The Great Banshee Switch
From Halopedia, the Halo wiki
The Great Banshee Switch is a trick performed in Halo gameplay using two airborne Banshees. It involves players switching their respective vehicles in mid-air.
The stunt requires great skill, timing, and sometimes just pure luck. First, both players must be flying in the same direction, and at the same speed. The first player must position his Banshee directly above the second, and just a few centimeters forward. At this point, the Banshees should be touching. Now, both players must get out at the same time, and when they are close enough, they must press X to get in the other player's Banshee. If done correctly, you will switch Banshees in mid-air.
This technique can be used if your Banshee is about to be hijacked. Just barrel-roll upwards as you are about to be kicked out and you will be flung upwards. You may then fall down on the Banshee and hold 'X' to re-board. A way to counteract this is, when you see them flip upwards, then immediately boost when you get inside, then the banshee will be out of the way before they can re-board it.
This trick is also useful for players in campaign. When you are in an fight with other Banshees, simply fly over an enemy's Banshee, press X, try to fall onto the enemy's Banshee, and then board it. This, however, is the most difficult method, as the enemy will often turn and flip a lot as to get in position to shoot you.