Originally Posted by TDC20
Also, has anyone ever tried to keep the action closed on a Glock during firing by applying pressure to the back of the slide?
Yes. Not with my thumb, but with the heel of my left hand. It's not difficult, and isn't likely to hurt you. It doesn't take much to keep it from cycling. You can do the same by wrapping your hand around the front half of the slide and frame (like in a self defense situation).
Any Cal is correct that these are recoil operated locked breech actions, not blow back, but you can do the same with a blow back action as well. (I've tried it in a 45 blowback, but not a 10mm.)
With my suppressed .22 pistol also, I sometimes hold the slide closed with my left thumb to make the shot quieter.