I didn't think it was, but I saw this one ref kept warning this one boxer when he kept crouching down to put more power behind his punches. Is it against the rules because he could kill his opponent by accident? Or is the ref a bad ref?
Bending with your head below waist level is illegal because of potential injury to the other fighter from low-blows and potential injury to the croucher because it increases likelihood that they will be hit behind the head.