That's an old clip, if you'll notice the rifle is larger than normal and thus carries a mean kick.
This clip came from a site that had videos of others attempting to fire the rifle while holding their balance, of about many guys who fired only a few remained standing.
I'll have to look for the link, if I find it I'll post it.
BTW - The rifle is a Bolt Action .577 (Yes thats right .577" Caliber!!!) called the T-Rex. Used mostly for Large Game Hunting in Africa, elephants, rhino's, etc.