Crouch jumping
From Halopedia, the Halo wiki
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.
You can also while mid-air hit crouch repeatedly to gain more air time, but not necessarily more height.
