I'm sure you know about rocket-jumping (although there may be some things you don't know yet.) These days, almost everyone knows you can do something similar with grenades too, or even combine the two explosives. But maybe if you know a little more about the physics underneath, you'll see that they are only one flavour of a more general phenomena within Quake.
In Quake, most damage comes from a specific location: the origin of some other quake entity. There are a few exceptions (such as damage from liquids like lava and slime) but in the main some particular thing in the world is considered to have damaged you. It might be a nail, or an explosion, or a monster, or a crushing wall. But whatever it is, the centre of it is somewhere in the world of Quake relative to you. And whenever this happens, you get a velocity boost away from the entity (nail, explosion, monster, wall) that damaged you. To be precise, the magnitude of the boost is equal to the number of points of damage inflicted multiplied by 8 pixels per second.
Normally you don't notice this very much. There are two reasons for this. One is that you are normally hit from the side whilst you are on the ground. This means you get a horizontal velocity boost which is quickly absorbed by frictional forces or your own movement. The other reason is that most of the things that damage you don't do that much damage. So you get a little push and recover very soon. Maybe you've noticed this happen when you are slashed by a fiend, or shot by a DeathMatch opponent.
Rocket-jumping turns this little feature into a very useful tool. If you are in the air, velocity changes are much more noticeable, because there is no friction to slow you down. And a nearby explosion does quite a bit of damage. (Er, you may have noticed that when you are playing DeathMatch. ) This gives you a pretty big boost; 50 points of damage gives a boost to velocity of 400 pixels per second. So create an explosion underneath you, and you get enough of an upwards velocity boost to go flying into the air. The only reason the rocket-launcher is so commonly used for this sort of jumping is that you have a lot of control over where the explosion happens relative to you, so it is pretty easy to hurt yourself with it. (Er, you may have noticed that when you are playing DeathMatch too. But this time it's a good thing, OK?)
But with skill and practice, other sources of pain can be used instead
when there are no rockets to hand. In
Quicker, we made use of
grenades, exploding spawns, radioactive boxes, a pouncing fiend,
Quadded-up versions of our explosives, and sometimes combinations of
these; and the current project, Scourge done Slick, will
take this sort of trick to new heights. As long as the pain comes from
below, there's a nice upwards gain for us in return. The sky's the
limit with these boosts. (Well, actually the variable
SV_MAXVELOCITY is the limit: nothing goes faster than
2000 pixels per second. But you'd need more than a Quad-rocket-jump to
reach that sort of speed.)
The good news is that id Software are reportedly building these sort of short-cuts into Quake 2 on purpose.