Boop!
This is a demonstration of "conservation of momentum." (Plus a wee bit of friction, inertia and angular momentum.) This is a famous demonstration given in every beginning physics course at MIT by Professor Walter Lewin. If you like this, there's hundreds more like it at YouTube. I'm glad to see NDT doing it here, although I know for a fact I'd flinch, myself.
@TheStorm You know the producer was fretting offcamera that insurance wouldn't cover this shot if it went horribly wrong because NDT was obviously doing it on purpose.
I think he just likes smelling cannonballs. because he's a SCIENCE LEFTY GLOBAL WARMING EVOLUTION PERVERT