You can increase the accuracy of your gun sights (which results in a zoom effect) by applying more pressure you to the pad trigger, but your movement speed will take a hit the further you zoom in.
There's usually no need for precise aiming when shooting opponents at close range, so in these cases you'll want to apply the bare minimum of pressure so as to maintain your movement speed.


If you're on the lookout for enemies it can be useful to hold the trigger down slightly to shorten the time it takes to get your sights up, while still keeping your movement fluid.