They are supposed to stimulate the muscle to help it relax. Physios use them alot to help with recovery of soft tissue injuries. I had them on my shoulders 2 years ago after a car accident. My shoulder muscle had tightened and were just not loosening so the TENS was used to help them relax again.

Bascially it's a little round rubber disc, you put it on the skin with that gel they use for ultrasounds & generally put a heat or cold pack over top as well. Then you turn it on & it feels a little like pins & needles. You start off with it fairly gently & can increase it as your muscle gets used to it - but if you go too fast the muscle starts twitching which can be uncomfortable. I don't think it's recommended for more than an hour at a time.

Oh oh, have you seen those infomercials where they use the machines to give you muscle tone & you don't have to do anything? They look like the muscles are spasming with them on? That's a TENS machine (not sure what the TENS stands for though!).