Quite often this is caused by shooters who partially insert the magazine, remove the hand, "here's the windup.......and here's the pitch!!", thus hitting the magazine into place. Some of that energy gets transferred to the frame giving it some inertia. That inertia of the frame allows it to move in relation to the slide, allowing the slide stop lever to disengage and then the slide goes forward to (most often) chamber a round.
It helps if you cup the palm a bit, which seems most common. Then your hand seats the magazine and is able to strike the frame. Don't cup it too much or it won't seat the magazine. Too flat a hand results in a seated magazine but the slide not releasing (except sometimes).
I have seen the following take place with this technique:
Pistol loads as slide goes forward on a locked in magazine. Pistol can be fired normally.
Pistol fails to load (empty chamber) as the slide releases just before/as the magazine is seating.
Pistol loads as slide goes forward on a partially seated magazine, which falls to the ground as the first shot is fired.
Pistol fails to load as the slide does not release, often resulting in repeated beating on the bottom of the magazine.
Pistol fails to load as the slide does not release, nor is the magazine fully seated and thus falls to the ground.
The above situations are brought to us, courtesy of an agency who teaches this as a feature. We encourage our students to equate insertion of a magazine with a full manual operation of the slide, regardless of the status of the slide or anything it might do while loading. In other words, we don't count on the slide falling while loading, to load the pistol. Experience has shown that occasionally it will not mean your pistol is loaded. A bad score on the range, but potentially fatal in a defensive situation.
My 229R DAK won't release at all. The Glock 17, 18, 19, etc. will do it with a good "slam" on the rear portion of the frame, and seldom do it when intentionally striking the forward part of the frame at the front edge of the magazine well. This also happened with a Sigma T&E gun and a S&W 990L. I still hope to try it with an M&P, although factory reps are conspicuously absent.
