Crouch jumping

The crouch jump, also called "duck jump", or "High Jump", is a technique where one first jumps and then crouches in mid air. This will bring up your legs and, for some unknown reason, your entire body, allowing a little more height to reach higher places, such as better sniping positions. Also when you fall from heighs that will give you death this trick maybe useful instead of death you get spared.

You can also while mid-air hit crouch repeatedly to gain more air time, but not necessarily more height. It also seems in Halo: CE that with a one handed weapon, (plasma pistol or pistol) you will look like your are jumping with no weapon to another player (arms apart).