In order to see this glitch, you must play in two-player co-op, and player two must be dead. Play the level until you get to the part where you chase the Heretic Leader in the Banshee. Kill player two, then, if needed, kill all enemies. After that, locate the arrow that points to the place where you must go to continue the level.
Fly above it and place yourself against the roof; in other words: under the floor where there are Grunts with Fuel Rod Guns that attack you in the earlier level. Get out and go to a "support" beam. When you cannot go any farther, crouch and walk off the edge. You will fly up until player two's screen hits the invisible wall and will show a close up of the Arbiter. Then you can walk around.
You cannot come back and finish the level after you fly up; you have to restart at the last checkpoint. Additionally, it is possible to crouch while in the air.