By quick triggering one can shoot faster then either Rapid Fire,
or pulling the trigger normally. Quick Trigger is firing the gun using the muscles of the
arm to move the trigger faster then can be done with ones finger. There
are two common methods.
1) The player simply moves his arm forward and backwards having their trigger arm parallel to the floor moving it rapidly forward and back across the trigger.
2) The player holds his trigger arm perpendicular to the floor, and tenses his muscles so his hand "vibrates" on the trigger.
The goal with quick triggering both methods is to find the least possible distance one can move the trigger, and still have it shoot. Then do it.
While quick triggering one often the advantage at long range. Simply because of rate of fire. One may wish to switch to dueling styles once your opponent nears.
Many players use their chins or shoulder to maintain stability of the gun. The advantage of this is obviously that the shots are more precise, that is the shots fall closer together. Also if the gun is on your chin and your eyes are straight forward, whatever direction your looking is also the direction your aiming. However Placing the gun on your chin or chest slows your ability to switch to a dueling stance.
While a good portion of quick triggering is about rate of fire it is also about being able to change between multiple targets, have good long, short range accuracy, as well as a good "startup rate". (Ones startup rate is how fast one shoots when one just begins to quick trigger).