Think a moment about what takes place when you start to pull the slide back... the barrel drops and begins to cam downward. When you press down on the barrel block, you're making it move on the cams backwards and down, like it's designed to do.
If you had the hand strength of the incredible Hulk to easily over come the recoil spring tension, you could probably do the same thing to a full sized M&P, or any other pistol... the smaller Shield is just easier.
Out of curiosity... I just unlocked my unloaded Glock 22 by pressing down and backwards (with moderate effort) on the top of the barrel block. I'd try it on my full size M&P's but at this hour of the night, I'm too lazy to go pull them from my safe.