So Resident Evil 6 has a lot of systems in place that it never ever explains. One of them is the Quick Shot system, which seems inspired off Call of Duty's left-trigger-feathering-aim-assist-system.
If you've played the game for a bit you might have found this out for yourself, but I noticed that if you press the Aim and Fire button at the same time, your character will pop a shot off from the hip, which might seem a little strange at first. However, if there are enemies within a certain range, your character will turn to face them when shooting.
In the case of Leon and his dual handguns, if you keep holding aim down and keep shooting, he will do a unique animation.
I haven't messed around enough with Resident Evil 6 yet to figure out if it does any extra damage, stuns the enemies or has any extra critical effect, but it's a great way to focus fire on enemies who are about to grab you from any direction.
I hope someone finds this helpful.