Butterflying
From Halopedia, the Halo wiki
Butterflying is a glitch in Halo 2 and Halo 3. You need two people to do this. In Halo 3, one person has to jump on someone's head repeatedly while the other person keeps on pressing RB and B repeatedly. It "cancels" a reload and a beat down to a lock on sequence allowing you to fly. In Halo 2, you press B and X repeatedly with an Energy Sword, but otherwise it is the same.
