Was that in the piece on Multi-dimensionality? Ohh! I think it may have been that when you swing a sledge and hit a tire you really do not know how hard you are hitting the target and thus have no real gauge for power output. In the case of a hammer throw you have a pretty fair idea of power/work output because of the distance traveled (neglects flight path variables but pretty darn close).
I think this may have been when we looked at boxing/kickboxing and the difficulty in quantifying work output while hitting and kicking something. Even D-ball slams are difficult in this respect as one can loaf on the slam portion and still have a rapid cycle time.