I had to use a pair of these during rehab last week. They were weird and interesting. Not for strength really, but for endurance and conditioning.
I had to hold them out like I was going to do an internal or external rotation. Then, I had to rotate in and out rapidly. The blade would wobble at each end. If I went too fast, the blade would move my elbow and I would stop and start over. I had to do 5 sets at 20 seconds each. My rotators were pretty tired from using it.
They are kind of expensive, but I was considering getting one for myself for general shoulder health (I can't figure out what else I'd use it for). Are there any other good uses for it?
Body Blade Website