From Halopedia, the Halo wiki
Cancel Reload is a button combination in the Halo games thats allows players to lower their weapon. Its most well known use is in Red vs Blue.
In order to activate cancel reload, press the following buttons in the correct order: X,Y,Y or B
During reload you might decide to want to open fire again, press Y before the weapon has reloaded and you can continue firing with the ammo left unless your magazine has no ammo left.