From Halopedia, the Halo wiki
From Halopedia, the Halo wiki
Melee cancelling is a set of glitches present in the first three Halo games that allow the player to cancel part of their melee attack with another action allowing the player to follow perform another action sooner. The methods and actions that can be performed afterward differ between games.
Halo: Combat Evolved
In Halo: Combat Evolved, melee attacks can be cancelled by throwing a grenade before the melee animation ends. Depending on when the grenade is thrown the melee attack can be cancelled before or after the hit. Aside from allowing for immediate grenade throws out of melee attacks this is also the basis for the original double melee]button combo.
In Halo 2 melee attacks can be cancelled with either the action/reload button or by switching weapons (x and y respectively in the original xbox version.) Both of these can be cancelled before or after the hit and in addition both can be repeated, allowing multiple cancelled melees to string together. This enables multiple button combos and techniques such as a double melee as well as butterflying. The action/reload method is particularly notable as the melee attack's animation will continue until it either finishes or is interrupted but once the button is pressed the player can interrupt the melee attack with virtually any action with no lag leading to further button combos such as the infamous BXR.
A unique property of melee attacks in Halo 2 is that the player is able to jump during lunge regardless of whether the player is on the ground or not. When combined with melee cancelling this allows for a movement technique in which the player jumps, performs a melee attack,, cancels the melee, and jumps with an immediate crouch. The result is that the player will fly over the enemy and have decent forward moment. This technique can be combined with BXRs by firing immediately after cancelling the melee. Doing this with a shotgun is particularly useful due to the shotgun's unique method of reloading. The technique is mostly useful in single player speed running. It does not appear to be possible in Halo 2: Anniversary's multiplayer but was distinctly made possible in campaign following the large update for Halo: The Master Chief Collection in August 2018 as well as the insider builds just prior.
The energy sword's lunge attack (its trigger based "firing") is also considered a melee attack in the game and as such carries all of the usual properties of melee attacks including the ability to be cancelled. Like a regular melee the lunge can be cancelled by either switching weapons or pressing the action/reload button and likewise can also be used in conjunction with jumping during a lunge. This allows for a much easier version of melee boosting and in fact is the more commonly known method of boosting. The height gained from jumping and sword cancelling with a mid air jump is useful for scaling certain walls and obstacles as well as clearing gaps that one would normally be unable to. Cancelling the lunge also allows one to avoid killing the enemy. Aside from allowing one to sometimes slide past enemies in campaign this is particularly useful when exploring multiplayer maps with others as it allows both players to get to locations as opposed to one dying and being left behind in the process. It is worth noting however that sword lunges will sometimes stop in mid air, particularly if the enemy is moving; thus it is best to perform it on a stationary enemy if possible. Mid air stops have been noted to be more prevalent in Halo 2: Anniversary.
In Halo 3 melee attacks can be cancelled by reloading one's weapon. This means that the technique is limited to using a weapon that can be reloaded and specifically the weapon must have at least one round fired off as well as spare ammo to reload with. This can be repeated and allows for butterflying in a similar manner to that of Halo 2. Prior to Title Update 1 this allowed for double melees but post update the melee attack could only be cancelled before hit. Sword cancelling is also possibly in Halo 3 but can only be performed by switching weapons prior to reaching the enemy, allowing one to avoid killing them. The lunging player is however guaranteed to stop when they reach the opponent and the technique is not repeatable thus limiting its overall usefulness.