This glitch only works on Halo 2 multiplayer.
On Ascension, have one person get the Banshee and the other person get the overshield and then stand at the Banshee spawn point. If the Banshee pilot boosts just at the right time when the other person jumps, the second person will be knocked off of the ledge through some clouds, then land out of the level but you can never get back except respawning.