Ive read a bit about about power slaps, and I know Peter Consterdine is a big proponent of it. Not sure the context of this video, but it definitely shows the effectiveness of a slap. Dont know if its intentional or not, but the guy in the red shirt has his off hand close to his own chin, I saw Peter C do this on a video, as a kind of disguised guard.
I use the "power slap" or kote-uchi in place of hook punches pretty routinely in training--they are quite effective! Obviously, this video proves that, but I've also seen other videos of the "power slap" being used to knock people out. My favorite has to be Bas Rutten's use of it in King of Pancrase, though!