Now, however, martial artists in particular have proven they can be quick enough to strip a gun from an opponent even a foot or two away. Even if fully cocked and ready to shoot, a quick enough person can disarm a gun toting opponent. This would also conceivably include an opponent behind the martial artist. And, having a gun to someone's head is rather stupid. It opens up the gun holder to be easily disarmed or allows the 'hero', in this case, to simply duck down, turn around, and execute a blow all in one quick and fluid movement. The gun holder has to be able to react first, thus forcing himself to lower the gun, then has to lower the gun and then make sure the shot will hit. Its already too late when the hero reacted, to even try and retaliate with the pistol.
I would not describe that as simple or easy, even if a few notables have done a trick under controlled circumstances.
Holding a weapon against someone with pressure allows faster reaction time (touch is processed by the brain faster than sight) and also allows one to sense movement even when not looking at them (the classic distraction).
Cocking a weapon manually also allows a quicker, lighter trigger pull, and thus a (very slightly) faster reaction time. Tiny fractions of a second.
However, a couple notes on the trope: first off, never point a weapon at anyone you aren't intending to shoot. If you have a gun drawn on an opponent and are telling them to surrender or you'll fire, there is no purpose in moving into close melee range; they struggle, you fire, you don't wrestle with them.
Expanding a bit on Inkidu, as the terms are actually very explanatory once you understand them:
Single Action: pulling the trigger performs one action - dropping the hammer onto the bullet to fire. This requires a manual "cocking" of the hammer. This method is pretty archaic; the best examples are pistols in cowboy movies (watch Clint Eastwood or John Wayne perform a "fanning" motion over their gun while they fire quickly - watch this
; at 1:50 you can see a close up of Clint fanning as he guns down several villains).
Double Action: pulling the trigger performs two actions - pulling the hammer back and chambering a new round to fire, then dropping the hammer down to fire. Almost every modern handgun works this way. As I said before, you can manually cock the hammer on most weapons, which will save a tiny bit of force and time; this is usually only used in accuracy shooting, though, and then mostly to minimize the pressure required to fire (which can pull a weapon off target).
Automatic: the weapon automatically performs two actions - a double action, but the back pressure of firing the round is used to cock the weapon instead of finger pressure on the trigger. This leads to the rapid fire of a machine gun.
Semi-automatic: like the automatic, it uses the back pressure to chamber a new round, but doesn't fire it until the trigger is pulled. Note that single action can be semi-automatic - although it'll have to be manually cocked to start with and again if there are any mechanical issues (if the back pressure doesn't cock a double action semi automatic, it will simply have a longer trigger pull and cock from the force of the finger pressure).