Re: how to release the slide.
An acquaintance and instructor who spent several years with Special Ops troops at Ft. Bragg, training them and helping them improve technique, told me that the U.S. Military long ago changed it's handgun training, and using the slide stop is the method of choice.
The change came about during the first few years of combat in Afghanistan and Iraq, when a lot of troops found their guns not going properly (fully) into battery following the slide being released. Part of the problem may have been due to the fact that the climate and environment often made it necessary for those using the handguns to wear gloves (due to the cold or because of the ough, heavily graveled rocky terrain).
While I've got 15 handguns, only one or two of them allow me to easily release the slide using my strong hand thumb -- and as one of the videos shows, that may not always be a good idea. The handover method (called "slingshotting" in one of the vides), COULD, with a Beretta M9 (or any gun with the safety/decocker leaver on the slide, lead an unintended decocking of the weapon. The US training was changed, but I'm not sure that unintentional decocking alone led to the change.
The old argument that slingshotting a slide was a gross motor, while using the slide release was a fine motor skill was disproved in combat, and by folks who studied the methods more closely They're both fine motor skills -- and if you don't release the slide cleanly using either the old "slingshot" (grasping/pinching the slide from the rear) or the hand0ver release, the slide is still not always going to go cleanly into battery.
As shown in the earlier videos here, using the off hand to release the slide is potentially a more efficient technique, requiring less hand and arm travel, and not necessarily slower. And. when competing, or facing someone shooting at you, you don't have to move the gun as far away from a potential target or attacker.
When I was shooting competitively, I got to the point that I used the off hand and several fingers like a small claw as I finished inserting the mag, to release the slide. The way shown in the videos, using the offhand thumb AFTER the hand has returned to the normal "support" or shooting position makes more sense, and is obviously a more efficient technique than the one I used.
Using the old slingshot technique, in which you grasp (pinch) the rear of the slide like you do the pocket of a slinghot (with a pellet or rock loaded in it) just doesn't make sense -- as you have to position both the gun and the offhand to get it where you can bring proper forces to bear on the gun and slide. And while you're doing that, the gun is much farther away from the target/attacker.
Using the handover method with other guns may be work well -- just not with a Beretta 92/M9.