It was a motor with plenty of nitrous going in. On that kind of engine if you let off the gas at all, the engine blows itself up. Maritan is right in that the rider was more or less fine, I don't remember whether it was his glove or just an engine cover that came off though.
yea when i first saw it i thought his hand came off too. the dude it happeend to posted on some forum i was on to clarify said he was spraying nitrous and there was some sort of explosion and it pushed the tank up and blew his glove off and it also hit him in the chest and cracked his sternum iirc.