A term for a guy masturbating. It is based off the MMA strike of the hammer fist. The act is highlighted by short, mini "strokes" in a very violent matter and would tend to injure most guys. The loose grip allows for the act of stroking as a normal hammer fist grip and strike would nearly be impossible anatomically and would probably not be pleasurable for any guy.
Dude, I walked in on Dave in the bathroom jacking it and I swear he was using the loose grip hammer fist!