This is yet another stolen idea from the past. Nautilus used to make a piece of equipment in the 80s called the "SportsMate", I believe, and it was for pronation/supination. The piece was mounted on the wall of the gym, and would always break, because some moron would essentially try to do one rep max work with it (the idea of the piece was one arm working against the resistance of the other).
I was just a kid and getting serious about tennis at the time, and I swear that the use of the SportsMate helped me develop strong wrists/forearms.