From Halopedia, the Halo wiki
A button combo is a sequence of buttons that, when pressed in order, results in the unlocking of some hidden feature or the execution of some glitch or exploit. In the Halo community, the term is typically used to refer to quick button sequences in Halo 2 that result in exploits.
Typical button combos take advantage of unforeseen attributes of certain actions. Some actions, such as meleeing, can disrupt animations for firing and reloading weapons, performing melees, etc. By chaining these and other actions, players can perform special tricks, such as automatic Plasma Grenade sticks and instant close-range kills. Professional players often favor these.
- "By the letter of the law, button glitching is cheating, and we suggest people to report it via feedback."
- — Tom Gioconda
The following is an explanation of how single buttons can be incorporated in button combos. For more information on their normal usage, see Halo Controls.
The X button will reload a weapon, but besides that, it can cancel the timers of animations, such as melee or grenades (off-host), enabling you to fire or melee sooner than you would normally.
The Y button will switch weapons, directly after which you can throw a new melee (or sword lunge). Besides that, it can also cancel reloading, cancel melee animations, and, when used in a quick double sequence, it can enable you to open fire again sooner after reloading.
The B button will let you melee, but when used at certain times it can do more; after reloading, for example, you can cancel the reload timer by meleeing and pressing X afterward. The melee's lunge can also be used to swing at your opponent, after which you can snipe someone no-scoped or stick people with a plasma grenade.
The only function the R button has is to shoot, but in combination with other buttons, a lot of tricks can be achieved.
Besides getting grenades out, you can actually use L to reload. After you have thrown a grenade, press X; you will automatically reload, but you can keep shooting while you reload (off-host).
Overheated plasma weapons have extra abilities. After a weapon overheats, it will reset the timers for shooting, meleeing and grenading; this can be used to get quick sessions of double grenades.
Button Combos in Halo 3
Button combos were never intended to exist. During Halo 3's development, Bungie treated such combos as bugs, and worked hard to eradicate them from the game. Some players, however, discovered a "BXB-style glitch", that allowed them to melee twice in quick succession.
Bungie quickly discovered this glitch and posted on the February 8, 2008, Weekly Update that they had deleted it from the game. It was fully removed once the first Auto-Update took effect on February 19, 2008.