putting a guy in the air!

I'm betting it's probably due to the fact that boxers are trained to keep their bodies low and coiled up like a spring... so when you hit a guy just right with enough force their bodies will sometimes uncoil and jump up like a spring. I think that was the case with Foreman - Frazier because Frazier always had that low, squat stance.

And with Alexander - Urango it was more of a case of Alexander catching Urango while he ws springing up into a punch which is Urango's custom.. so the punch simply continued Juan's upward momentum and allowed his body to spring off the canvas. I'm prety sure Alexander did that to Urango more than once in the fight it's but only once did it cause a forceful knockdown.