Quick switch is a button combo.
Button Combo Edit
Y, Y, Y or Y, B, Y, Y or Y, B, X
If you want to be the faster in the combo, after releasing a plasma bolt, just switch, melee and switch switch (or cancel melee by pressing X) and you'll be able to fire a split second before you combo-opposition can. Also Y, wait, YY works.
Note: All rifles allow you to fire straight after the Quick Switch and Quick Reload, but all heavy weaponscan fire just as soon as they usually can even though the animation finished. This is only possible in Halo 2's multiplayer.