i have one for my shoulder, it doesn't like my back cause i forget i have it on and it tightens up.
what do you need to know about them andy? hooking them up to your legs is kinda interesting if you have pants on.
they have little pads that you stick on you and mine has 8 different burst modes and 4 alternate ones on the top of it, and than there is the speed. you can control how long the bursts are and how many zaps per second you get. i have mine set on 30 zaps per second for a "massage", but i only use it on my shoulder. gotta be careful using them because they feel good when you use them but if you leave them on too long, they stimulate the muscle too much and you hate life later.