This topic comes around now and then and I've read several threads, but I just thought of a new way to think about it that makes sense.
I've read enough threads and done some calculations, to say that a 45 acp has knockdown power similar to an 80 mph fastball... which isn't that much "knockdown power". Knockdown power is generally a myth.
The new way to think about it though is "for every action, there is an equal and opposite reaction". If you fire a weapon that knocks you back on your butt, then yes, that probably has some real "knockdown power". But that's going to be something like an elephant gun, that is hard to manhandle the kickback.
Just thought I'd share. Knockdown power makes more sense to me thinking about it that way. Cheers!