i remember seeing this ko a few times. each and every time with a southpaw.
you ever saw a guy get struck out by really nasty break?
my theory is you think a punch is coming out straight.
but it comes out curved and weaves around on you.
the punch curves and "breaks away" so to speak on where you think it was going so your brain does not understand why you were just hit and has to momentarily shut down.
michael nunn knocked out sumbu kalambay the same way with the same punch.
same ko. same "breaking" punch. but thats just my theory.