Strafe jumping is one of the many physics
bugs come
exploits in
Quake (and other
first person shooter games) that was one of the crucial components to both the game's success and longevity. Like the (in)famous
rocket jump, strafe jumping was unto itself a
fine art that allowed for tremendous mobi6lity and rapidly altered all forms of play. It was a technique developed from
zigzagging and eventually spawned
bunnyhopping.
The basic premise was that by simultaneously jumping, turning, and strafing one could gain a higher than normally attainable speed while in the air. While travelling in the air, ground friction is inapplicable, so the speed was maintained until landing. This allowed for both faster movement and larger jumps to be made. The former, unless executed bunny hopping style, was mostly ineffectual, but the impact of the ability to perform larger jumps was significant, allowing players to jump previously uncrossable chasms and perform incredible maneuvres.
As it is a far more complex technique that it sounds - the speed and magnitude of the turn, the timing of the jump, the amount of strafe are all factors that contribute to a good strafe-jump - it greatly broadened the concept of good movement in what were previously aiming-orientated games.
It is evident in a variety of first person shooter games - all forms of Quake - to varying degrees. Generally the effect is much the same, allowing for greater jumps to be made. It is hard to emphasize the significance of such increased mobility in a first person shooter game. Important to note, however, is that the impact of bunny hopping, a technique directly derived from the strafe jump, is much more varied from game to game.
The ability comes from the acceleration made while turning (even at full speed - acceleration is after all a change in velocity which is both a vector and magnitude). This is compounded by strafing, and jumping means that the speed cannot be simply decreased by ground friction. It is possible to prevent strafe jumping, but it leads to an extremely unnatural feel to the game - Carmack documented this in his .plan while developing Quake 3 Arena and thankfully decided to retain it). Serious Sam is one such example of a game without strafe jumping.