Crouch jumping

Introduction
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 heights that would normally kill you, if you manage to crouch while falling, you may survive. In Halo 2 and Halo 3, however, the damage reduction for the crouch jump is unnecessary due to the lack of fall damage. In addition, if the fall is too long, then even a crouch jump won't save you. For example, if you jump off Gephyrophobia, you will not make it, despite your crouch jump, because of the extreme length of your fall.

Performing the Jump
The method of crouch jumping is relatively simple. Press 'A' to jump and while in mid-air hold down crouch. This should enable you to get to previous unreachable places. The reason this trick works is not because it makes you jump higher. What actually happens is that when you do a normal jump, occasionally the place you're trying to jump to is too high and your feet hit the platform edge, thus not allowing you to get to where you were trying to go. When you crouch jump, your feet get "tucked under your body", so instead of hitting the platform edge, your feet land on the floor and you can now access higher platforms.

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