Apparently, it's not just the magnetic effect, it's the concentration of the bio-electric activity in the body. The copper of the bracelet seems to act as a booster antenna. I recently met a friend with a rotor cuff problem and became interested in what can be done for recovery. From what I've read, keeping the muscles relaxed is important. I haven't heard of magnets being used for pain relief (it might be that magnets might irritate the muscle), but rare earth elements are used, probably to concentrate bio-electric activity.
In theory, it is logical that magnets concentrate the minerals in the blood to benefit the effected area, but the ionic effect of the electromagnetism might also be similar to the alkalizing of the pH balance. It seems to be a traditional treatment more in Japan and Asia (see link below).