To perform the Perpendicular Banshee Fire Glitch, you must have Halo: Combat Evolved. Point the Banshee so it only goes straight up, then fire its twin plasma cannons or fuel rod. Notice that the shots go perpendicular to the direction your Banshee is going, not forward like it usually does.
On the level Two Betrayals when you hop into the Banshee and fly back into the control room, nose-dive into the large pit until you hit the bottom. Then get the Banshee to look directly up, when you start flying upward your fire should go to the side. When you get out of the pit however, the glitch will stop and the Banshee will fire normally.
This works with Halo PC, in any Multiplayer map that supports the Banshee. Just fly up until you reach an invisible wall and fire the Banshees guns.