Here's some info on it-
http://www.sci-therapies.info/FES.htm

I use a EMS{electronic muscle stimulator} for my upper back, shoulder and arm muscles when I need to.
It's an inexpensive little thing, but it helps to work the muscles or relax them depending on the strength of the settings.

IF {Interferential} stim is better than the EMS stim- more power can be used without the slight stinging at the pad placement areas.

There are also many other types if electronic simulators, I don't know if a specific type is used for the FES.