From Halopedia, the Halo wiki
After a certain time in a reload animation, the weapon is reloaded. However, this combo allows players to cancel the reload animation and immediately fire their weapon again. More specifically, pressing Y switches weapons and cancels the reload animation; double-tapping the button switches from and then back to the current weapon. This also works to cancel out the Energy Sword's "reload time", since there is a delay after a sword lunge.
Players lacking a secondary weapon can use the combination X+B+X. The combination works similarly in such an instance, with B (melee) canceling the reload animation and the second X press canceling the melee. B should be pressed just as a new magazine or clip has been inserted into the player's weapon.