From Halopedia, the Halo wiki
To perform this trick, play Co-op campaign on the level Assault on the Control Room. One player needs to stay in the Pelican and the other leaves the Pelican. As the Pelican flies away, Player Two commits suicide, the Pelican will reach a spot where player two will respawn, the player should land on a small rotating ring. Player one will then proceed to kill him/herself, they should land directly behind player two. To stay on the ring, the players must continue to run forward.