I've seen a LOT of videos of military and police training of dogs. Mostly these are Belgian Malinois/German Shepherds. They are trained to go for the arm when their trainer is attacked.
And if he was right-handed, John would have instinctively raised his right arm to his face, which explains the pattern of wounds up and down his lower and upper arm. Not dancing, magical taillight pieces that his arm was incapable of shattering in the first place.