Nitrous back fire. Blew the top of the air box off and lifted the tank. The rest of the explosion traveled out of the front of the airbox and into the ram air ducts. The ducts are two pieces glued together. The top was was blown off and hit the guys arm, thats all. Keep in mind a piston (290 grams?) would have to travel through 4-5 inches of aluminum head and through the aluminum frame to hit the guys arm. It would also have to do it on the upstroke.