I have been experimenting using the TENs unit on the genitalia alone at first then introduced them into scenes with my Mistress.
My experience is they are terrific, Now I use the term TENS but there are many available,
I happen to have been given a TENs unit, so that's the only one I know.
I have found having the electrodes attached does wonders for making one a motivated enthusiastic obedient submissive.
I now work with two of them and it is amazing, and they also work great for hands free, alone and amazing results.