GUYS GUYS GUYS.
This is a rube act. A joke ride. It's circulating facebook and COTH too. I didn't realize it either, until somebody else pointed it out to me.
Watch the video again, and carefully - he loses his stirrup twice and both times his position is perfect.
Especially watch the slow motion. His leg stays into position then he kicks it back after the horse's motion. He doesn't slam the horse's back even once - watch it carefully, and you'll see that he puts his weight into the stirrups and actually lands softly. He never once bangs the horse's mouth.
Believe it or don't. The horse has nothing to complain about, the rider is a true expert.