I think he means the top of your hipbone. That's where I try to place my elbow when I cover up and it works well for me.
Pedro Pena could take huge body shots. Two insane hools once tried to jump Pena and covered him with bare knuckle body shots - useless.
Question- how does the repetitive hitting with weighted medicine balls mechanically help strengthen the core? Is it mental endurance, or is there a physical component in toughening the torso as I would assume?
Getting hit in a contracting muscle will cause more muscle fibers to fire making the exercise more intense thus building muscle more completely. It should help strengthen/thicken the linea alba (a tendonous sheath behind the rectus abdominis) and it will cause a faster response from the muscle spindles (what makes the muscles contract in response to impact).
Trouble is while doing it, the reproduction organs get bruised. Therefore having to much to worry about repairing themselves, than working on what they are there for in the first place.