Once you commit into a tackle everything starts to move and it won't stop just because you mistimed the tackle. In fact if you stop moving the arm it would have looked like a shoulder charge which carries severe penalty. Of course he knew it was coming, you can hear the ref talking, he's not whispering. I can't remember what he would have hit if the arm didn't go down. Maybe he was trying to pull out, you can see the arm rotating, to prevent direct contact with 10's head.