From Halopedia, the Halo wiki
Flyboy is a glitch in Halo 2 that allows players to store the lunge of an Energy Sword while in the seat of a vehicle. It is also known as "lunge storage" and has also historically been known as "sword flying." The glitch is only possible on the original Xbox version of the game. It was removed in Auto-Update 3, which is also included with the Halo 2 Multiplayer Map Pack, though it can be restored by clearing the console's cache. The glitch works in both campaign and multiplayer.
The glitch is most easily performed by activating an energy sword lunge (on a target within range) while sitting in the passenger seat of either a Warthog or Spectre. Though more difficult, the glitch can also be performed with any usable seat of any vehicle by activating the sword's lunge and immediately getting into the vehicle. The sword lunge will be "stored" regardless of whether the lunge target or the vehicle moves. The lunge can also preserve if the player manages to exit one seat and immediately enter another.
This glitch results in two effects. The first is that when the player exits the vehicle (without reentering it immediately), they will automatically fly in the direction of the stored lunge, typically to a much greater distance than the initial lunge. The second effect is that in campaign, the player will be immune to the dying from falling too fast for as long as they are in the vehicle.
The first effect gives it similar usage to Sword flying, as both glitches allow players to travel through the air quickly and to locations usually not traversable, though some of the more elaborate Flyboy setups allow for distances and directions that are impossible for normal Sword flying. The two glitches can also be combined to allow storing a lunge from a greater distance. However, when using the passenger seat of a vehicle, scope weapons cannot be used for Sword flying as the scopes do not function.
The glitch is possible to perform with a normal melee attack as well. When done this way the player will still have immunity to death by falling too fast. However it does not appear to grant as large a lunge and is more difficult due to requiring the player to be much closer to their lunge target as well as requiring the target to not be a vehicle or in one.
Start a multiplayer game with at least two players on opposing sides on a map with both a Warthog or Spectre and a Banshee (Coagulation is recommended). Have one player with an Energy Sword (Person A) sit in the passenger seat of the Warthog or Spectre ; the other player (Person B) should fly over them with the Banshee. As Person B flies over Person A, A should aim at them and attack with the Energy Sword when their reticle turns red. Person A should then exit the vehicle, which will cause them to launch through the air in the direction of their initial lunge attack.