Remember the video Patrick did about the unfired Knights Armament Suppressed M9?
It has a special lever that locks the slide and prevents it from opening. Well this is not the only gun to have done it. S&W’s Hushpuppy does the same thing. Looking at the photo above and below, you can see a triangular cut in the slide and a modified slide release.
This locks the slide from reciprocating and makes the pistol as quiet as possible.