I believe there are more than one type of TENS unit. I have an over the counter type, and it can be effective.
Your Dr has probably ordered one of the more professional types, that can be programed by them.
I think they send electrical impulse to the muscles, and will constrict and relax the muscle, there by, adding strength to it.
Actually, they can be quite soothing, and tend to take your mind off the pain, as long as your wearing it, and it is going through a cycle.
I'm sure there will be someone along who will be more knowledgeable about