Shit, he's not even tensing, if he was his thumb would be inside his fingers and his fist would be turned towards his bicep. Seriously, do it now (if you can bear to look at your own bicep right now) - see how much tighter it gets when you turn your fist inwards. He doesn't even need to, it would probably just break straight out of the skin.
kyle is quite possibly the most well rounded rider in the sport. he is the only guy who can do world cup races on a saturday and then hop on his hardtail and do double tailwhips on sunday and then cap it off tailwhipping 20 foot drops on an sx trail.