Glock's Trigger (Return) Spring
Here we go: First, the often stated primary purpose of Glock’s, ‘trigger spring’ is to help you pull the trigger; AND that is how all the Glock aftermarket parts companies sell this spring.
However, all of us need to realize that one of the two principal functions of Glock’s, ‘trigger spring’ is to act as a form of, ‘mechanical guarantee’ for the sear’s, ‘kick plate’. This allows the trigger bar, itself, to make: regular, firm, and consistent contact with the striker’s lug each time the Glock fires.
A Glock’s, ‘trigger spring’ does two important things: (1) It helps the shooter to pull the trigger; and (2) it helps to raise the trigger bar back up into its reset position or, perhaps more correctly, to hold the trigger bar in firm contact with the striker’s lug.
This is the action, and this is the sense in which I tend to think of Glock’s, ‘trigger spring’ as a, ‘trigger return spring’.
Now, what is the primary mechanical force that is responsible for resetting a Glock’s trigger mechanism? I won’t keep anyone in suspense:
It’s recoil - plain 'n simple good old fashioned mechanical recoil.
It really doesn’t matter whether or not a Glock’s trigger won't move forward (as in reloading). What actually matters is whether or not the slide’s backward and forward momentum is sufficient enough to, ‘jump’ the trigger bar back up into contact with the striker lug.
So, in addition to its initial function of helping the shooter to pull the trigger, a Glock’s TRIGGER RETURN SPRING, also, performs another EVEN MORE IMPORTANT PRIMARY FUNCTION: It provides mechanical assistance to the trigger bar's upward momentum in order allow it to more surely reset itself against the striker (FP) lug.
Regardless of what position the trigger is in, whenever the trigger bar is up and reengaged with the striker lug that Glock is going to fire. It’s only when you manually rack the slide, as in reloading, that the trigger bar will fall down and DUE TO THE LACK OF FORCEFUL SLIDE MOVEMENT (RECOIL) not reset itself.
This is, also, the reason, 'Why' it's a good idea to slightly elevate a Glock's muzzle while you're holding the trigger to the rear during a mechanically unassisted manual trigger reset.
STRICTLY SPEAKING, HOLDING THE TRIGGER TO THE REAR IS NOT SOMETHING THAT YOU ABSOLUTELY HAVE TO DO; BUT, IT HELPS!
Here’s a demonstration video by Randy Smith that clearly shows what I’m talking about:
ADVISORY: I am NOT a Certified Glock Armorer! (I'm only that armorer which Gaston Glock has forced me to become.)