Thanks go out to:
The SDA Staff (there are officially too many of you now) - for keeping SDA awesome!
Nate - for sharing my love of Sonic, developing tools to keep making the tech side of speedrunning easier, and providing the capture card used for this run

The Sonic Center - www.soniccenter.org - Still the most comprehensive resource on competitive Sonic gameplay anywhere.
stanski - for many, many strategies. He claims he sucks with Knuckles, but there are a bunch of times he has that I still can't touch.
werster - even more help with strategies; a lot of his stuff is not exactly SS-safe but its great to see just what is possible in the stages
Fuzzerd, Aglar, Upthorn, Sonikkustar - the game's TASing crew. Shoutouts to Aglar in particular for explaining the finer workings behind the more difficult tricks in this run
DMTM - for discovering the Casino Night 2 level wrap (this is actually an anti-shoutout because this trick sucks)
And last but not least, MrSparkle, for discovering the clip through the mesh in Metropolis 1, which turns a potential stumbling block into a relatively easy level.

After I finished the Sonic run I decided I had had enough of speedrunning for at least a couple of months. I was going to sit back and just play games casually for a while. The thing is, when the speedrunning bug bites you, that's not how it works. My desire to do this run was rekindled in a couple of different ways: firstly, I have to thank a couple of guys over at TASvideos, Felipe and skychase, who took on a project I had considered a while ago, completing this game with Knuckles while not collecting a single ring, and succeeded at it! I couldn't help trying out some of the tricks, and especially stuff like Casino Night 2 helped me become familiar with the more difficult stuff in the run.

Around this time as well, I was streaming this quite a bit, and consistently getting high viewer counts (up into triple figures some nights!) so I figured it was something that people wanted to watch. The more I thought about it, too, the more I reliased that this run would be significantly different from a Sonic run, even though the final times would be very similar. And with that I started working on the run for real.

One general thing about this run (and probably the one thing I'd change about this game if possible): unlike Sonic 3, when Knuckles drops from a glide in Sonic 2, even if you're holding down you can't charge a spindash until the "I dropped from a glide" animation ends, which takes 15 frames, or 1/4 of a second. So if some of my spindashes look slow, that's a possible reason for it.

Some terminology that I'm likely to use throughout my commentary:
* Frame - 1/60th of a second, the smallest unit of game time.
* Frame window - the amount of these units you have to execute a particular action. A "frame-perfect" action has a window of just one frame, for instance.
* x-tap spindash - Each additional button press in a spindash gets you more speed, up to a maximum of six. Sometimes having a specific amount of taps gives you the right speed to get through a certain area quickly or easily.
* Universal timer - There's two different timing properties of moving objects in this game. The universal timer is where an object starts moving the instant you start the level, and will therefore be in a certain position at a certain time, every time.
* Camera timer - The other kind is this one, which only starts moving once it's brought onto the screen (or near the screen - when the object is first rendered, in any case). Knowing which objects have which timing properties is important to good times when you have to deal with them.
* Ramp jump - Knuckles's jump speed and height are dependent on both your speed going into the jump and the slope of the ground you jump off. Running up a slope causes your jump to become much higher, while running down a slope will make you jump quite flat to the ground, gaining speed. This applies even if the slope is very small.
* Loop jump - Similar concept, but inside a loop. If you're moving through the loop from left to right, a loop jump is from the top-left to the bottom-left. It's kinda finicky because if you land at different angles you get different speeds, but in the best case it is faster, and it's also a very convenient way to switch from rolling to running.
* Superglide - Simply jump on an object (an enemy or an item box) and glide just before you hit it. This causes you to translate your falling speed upwards, and since you're gliding you have a much lower rate of acceleration towards the ground. In layman's terms, you end up a lot higher than a standard jump.
* Path switcher - This is an invisible item in the game that allows the levels to work as structured. There are three different levels of background/foreground and Knuckles is always in one of them at any one time, and these items seamlessly switch Knuckles from background to foreground and vice versa. This is especially necessary for things like loops to work, because if you didn't have them they would just be solid all the time and you wouldn't be able to enter it. Knowing where these are and how they work is key to some tricks in this game.
* Zip - A zip occurs when Knuckles is stuck in a wall somehow and presses left or right. Since collision detection is designed to stop you from going into a wall in the first place, pushing left against a wall makes the game push you back to the right so you aren't embedded in it. As a result, when you're already inside a wall holding left or right causes you to travel in the opposite direction much faster than you can attain by normal play.
* Level wrap - This is what happens when a zip causes you to bypass the left edge of the screen. The game treats your X-coordinate in memory as a number less than 0 - but position coordinates are unsigned, so instead of a negative number it's a VERY large positive one - which correlates to the end of the stage. In this run Casino Night 2 and Metropolis 2 are skipped this way.
* Screen wrap - Similar concept, but using Y-coordinates instead. If you can move off the top of the screen, the game thinks you're all the way at the bottom. This only works in certain levels which wrap around themselves infinitely - which in Sonic 2, is the three Metropolis stages only.

On to the run, then!

Emerald Hill 1 [0:20]
[0:06] About the only thing that differs from a Sonic run in this stage is that this jump over the spring is much tighter due to Knuckles's lower jump. Other than that it's just as straightforward.

Emerald Hill 2 [0:38]
[0:14] This is a slightly trickier level for Knuckles because of this ramp jump. What I need to do is bounce off the invincibility and carry that bounce across to the bee enemy up ahead. The problem is that Knuckles can't regulate the height of his bounce - once you let go of your jump, pressing any of them again will cause you to glide instead. Here I pulled the jump off with the correct height, but it's very possible to go flying way too high off the ramp, which means you spend too long waiting before hitting the invincibility and costs you a second.

Chemical Plant 1 [0:23]
[0:18] This is the one place where the speed cap is actually useful: Sonic just holds right and jumps straight into this gap, whereas Knuckles has to slow down and line it up.

Chemical Plant 2 [0:40]
[0:05] Yes, it's possible to catch an early cycle here by just jumping off this ramp and hoping for the best. However, that's exactly the problem, you're just hoping for the best, you don't really have any control. If you jump straight off the ramp there are no less than five different possible outcomes:
a) Land on the platform and continue running
b) Land on the platform and stop dead
c) Land slightly embedded in the platform
d) Land completely embedded in the platform
e) Fall through the ramp and die
Of these, a) and c) are the results that can get you to the early cycle in time, but they are both quite rare. The most common result is b), followed by e) which is obviously the worst. If you get d) you can zip to the right but the door will kill you, so you can't make the early cycle.
After e) started happening to me a lot more than usual I decided to just take the loss and do it this way. It's equivalent to getting b) or d) from the list above and gets more attempts through to Casino Night 2, which is the most difficult trick in this run.
[0:24] If you clear this platform you can save a second, it's a tricky jump with Knuckles just because of his lower jump but you can make it kinda consistently.

Aquatic Ruin 1 [0:23]
[0:04] Screwed this up a little bit, I got too close to the enemy and my right hand panicked a little and went into spindash mode early
[0:19] And then completely missed this jump up to the middle of the loop, which cost about two seconds. No big deal though.

Aquatic Ruin 2 [0:41]
[0:11] Superglide off this box -> make stage entirely trivial. It needs to be a pretty specific height though: too high and you bump into a pillar near the end, and too low and you catch the lower of the rotating platforms, and gliding gets faster the longer it goes, so there's no way to catch that up. High is fine since you can just stall a bit to lower your height (which probably costs a second), but if you're too low you lose probably four seconds.

Casino Night 1 [0:29]
[0:14] This jump is actually much easier as Knuckles because of the smaller jump. A full size jump from the shield box here goes straight through without hitting any of the bumpers (or hitting the last one in such a way that it doesn't matter).
[0:21] Knuckles can't make the jump across to the top in one go so it's slower than just taking this route.

Casino Night 2 [0:35]
Here we go. This is THE level, the reason why Knuckles is faster than Sonic, the reason this run took as long as it did, the reason Aglar gets such a shoutout in my thanks up above.
[0:04] OK, from the top. The first part of this trick is to do a full size jump off the top of this platform and do a frame-perfect glide to clip behind the speed shoe box. Too early and you have to climb back up again (this takes about five seconds, I normally gave this two attempts), too late and you break the box and the run's over.
[0:05] Now, before I go through the rest of it, if you're not familiar with TASvideos, allow me to blow your mind by introducing the concept of a sub-pixel. Yes, there are smaller units of measurement in games than pixels! In this game each pixel is divided into 256 sub-pixels, and having these in the right range in both the x- and y- planes is essential to this glitch working. I'm not even kidding. The good news is, you can manipulate your x-subpixel into the right position. The bad news, you can't do the same for the y-subpixel. So, once behind the box, I have to tap right for 2 or 3 frames, this causes the x-subpixel to be in the right place. Now I spindash and get pushed into the floor, and do two more quick spindashes. This is where I get to find out whether my subpixels are good.
[0:07] Obviously this is the intended result, but if my y-subpixels are no good I will be pushed down into a box in the wall that I can't escape from, and if my x-subpixels are no good I will be ejected much more slowly than this (not nearly fast enough to go through). About 3/8 of the potential y-subpixel values will not work, and it can't be manipulated, so I only have a 5/8 chance of pulling this trick off even with the correct input. It's the closest thing to a legitimately "random" element in any of the classic Sonic games.
But it's still not over! I also have to let go of right at the right time. There's two windows for this: one is the first two frames after exiting the wall, and the second is three frames just as you go off screen. I completely whiffed the first one but luckily paused in the right area for the second.

Hill Top 1 [0:40]
[0:14] There's a tiny part of this ramp that's much steeper than the ramp around it. Hit it and you get this jump all the way to the top of this wall instead of halfway up.
[0:23] Same as Sonic's zip, though I got really lucky here and ended up going slightly further than usual, allowing me to catch the platform instead of having to bounce back up off the spring.

Hill Top 2 [1:01]
[0:04] Probably the biggest mistake in this run, my spindash wasn't quite strong enough and I failed to clear the loop. If you're wondering why I didn't glide to try and climb up, you can't grab that part of a loop. Cost close to 4 seconds.
[0:20] This superglide is very handy because it means that you're not reliant on the cycle of moving platforms up ahead at all - the path across the top doesn't have any.
[0:32] This jump is way easier as Knuckles because of - you guessed it - that short jump again!

Mystic Cave 1 [0:36]
This is my favourite route in this run. I was streaming one night and werster pointed out a bunch of improvements and simplifications, so I ran the level over and over on stream trying to nail it down. It's one of the more technical levels in the run, and I'm really glad with how it turned out!
[0:15] This is the first change - Knuckles takes the top route here. This would probably be faster with Sonic too if it weren't for the damn spikes at the top of the ramp. From here you can spindash and jump, land on the box and jump again, to skip that pesky lower route.
[0:23] Now the second part. We used to superglide off the invincibility box on the lower level, but I realised that you could just use this enemy instead. It's a ton easier and roughly as fast - the only danger is if you glide to exactly the wrong height, you hit the bottom corner of the wall and drop off.
[0:30] Finally, instead of making the last bridge fall you can just climb up to the vine, saving about half a second.

Mystic Cave 2 [0:44]
[0:12] Knuckles can't jump up to this platform, so instead we get to see the only application in Sonic 2 of a damage boost!
[0:33] There's five different patterns I've observed for the way these spikes fall - as soon as I saw the one fall through the middle I knew that standing on the right was completely safe.

Oil Ocean 1 [0:36]
[0:08] The only mistake in the stage was here, didn't spindash for some reason. Apart from that this stage was pretty much flawless.
[0:26] From here, five full jumps gets you in position to quickly climb up this wall and spindash away before the fan kicks in. If the fan stops you you lose a good four seconds waiting for it to stop.

Oil Ocean 2 [0:55]
[0:14] Changed my strategy on this slightly, instead of trying to line up a 4 or 5-tap spindash I just do a full 6 and then slow down just before hitting the ramp. It has the same effect.
[0:24] Another slight change, Knuckles can't hit the spring in the background, since it's too high up, so he has to take a hit on the spikes and use them to get up.

Metropolis 1 [0:49]
[0:28] Shoutouts to MrSparkle! He found this trick to clip through the mesh cylinders. You can also do it on the first cylinder but it only saves about three seconds if you get it first go, and it's frame-perfect.
[0:40] If I had got here half a second earlier the last arrow would have been pointing up and that would have been bad news. But as soon as the second one retracted I knew it was safe to go.

Metropolis 2 [0:16]
This is very much the same as with Sonic, except as Knuckles you glide into the zip point, which is easier... unless you're at the end of a speedrun and like choking like I do. >:/ Again, once I activate the zip I pause to line it up. This one has a four-frame window to release right, so it's significantly easier.

Metropolis 3 [1:20]
This level was PERFECT for my route. Stanski has a time one second faster, which is doing everything here plus spindashing through the mantis at the start of the stage instead of jumping over it, which is about the most SS-unsafe strategy ever.
[0:11] Knuckles can't quite do this glitch in the same way Sonic does. Luckily, he has a much easier method.
[0:22] Holding one of the jump buttons causes you to go much higher up these. You can use this knowledge to take this completely safe path over the mantis at the top.
[0:40] This is much riskier than I used to do here but I have a consistent method to do it now. Once I jump off the screen I'm watching a set of flashing lights in the upper left of the screen, about in line with the numbers in the HUD. Once all three of them move off the screen I jump and the bolt pushes me into the floor. If you're too late on this jump, you die, and if you're too early you just appear on top of the bolt, costing 5 or so seconds while you set it up again.
[1:13] I changed the last two hits slightly, attacking from the left side instead of the right. The reason is that by the time Robotnik hits the bottom of the screen, the last sphere is on the right if you've done the rest of the fight perfectly.

Sky Chase [2:05]
As with Sonic, this stage is simply free points. I screwed up the combo towards the end but it doesn't affect my time so I don't care.
For some reason, this stage ends a few frames earlier as Knuckles, so you can scrape a 2:05 in. Why? Who knows...

Wing Fortress [1:56]
[0:09] I missed this jump over the chicken here, so I have to do the next area slightly differently. This means that the last platform I need to jump off is in a slightly different position, but I've practiced for it so I know what's going to happen.
[0:19] This is kind of scary because I don't know exactly where the conveyor platforms up ahead are going to be, and so I'm really nervous about starting the glide in case I tag the edge of one of them, break the glide and can't recover. I ended up getting the glide off, but I was quite low to the ground and, now I think about it, was very lucky that one of the platforms up ahead was up.
[0:27] I'm not entirely sure if the old method I was using on this stage is even possible with Knuckles (spindashing up the ramp and jumping off at the very edge, as in the Sonic run), but werster pointed out to me that there is actually a hook over here! Interestingly, this method is just as fast as pulling off the tricky jump.
[0:34] And somehow completely missed this jump off the zipline. Lost two seconds to that.
[1:05] This trick is actually just as easy as it looks. Hold down, one-tap spindash, jump, pass through laser. Knuckles actually saves time with it because, unlike Sonic, he can glide to halt his momentum and fall down the gap.

Death Egg [0:51]
[0:13] Contrary to what I thought earlier, Knuckles actually -can- trigger the infinite Silver Sonic glitch which locks the camera in the Silver Sonic arena until time over. He also has a second possible glitch, where after the 8th hit he doesn't lose his hitbox and can still kill you (or you can kill him again for 100 points), which I thought replaced the infinite Silver Sonic glitch, but I discovered while dicking around before an attempt session that that wasn't the case. So I just play this fight really safe and make sure that I don't kill him anywhere near the glitch frame.
[0:26] Every single one of these hits is a nerve-shredder at this point (even hit 2, which should be completely free - I've had Knuckles simultaneously jump into the boss and the flame and kill me before). Prior to this run I lost two (slightly worse) runs on hit 3 and hit 6 respectively. The relief once he flew up after hit 9 was immense, and my reaction after escaping was... slightly excited. :)

I have no doubt there is still improvement to be had in this run. My best attempt that got ruined late would have come out to 15:51, and with these strategies, Hill Top 2, Metropolis 1 and Wing Fortress have five seconds worth of improvement each on this run. If I hit the Chemical Plant 2 early cycle as well, that's another six seconds, putting the run into 15:3x territory. However, I wish whoever goes for that run the very best of luck. :)

Enjoy the run!