Khan was actually very clever with the shot, you see when he landed it, there were a couple head shots before it and a little clinch, I just don't think Zab saw it coming and so wasn't tensed at all and it probably winded him.

It's much easier to take a body shot when you can squeeze your muscles to firm them up. If your just breathing and thinking about something else, that's the ones that knock the wind out of you.