LOL. Common misconception - this is not mains we're talking about. The stim units are designed with specific frequencies intended to produce intensely pleasurable sensations. Ever had a hands-free orgasm that almost took your head off? Well, stimming does that for you.
Just to put things in perspective, most units are powered by a 9V battery. You're not going to get "shocked" by that
In a nutshell, and IMHO, an obscure and yet insanely pleasurable form of masturbation. In a world of Fleshlights and fake pussies, I'm surprised more people don't look beyond the obvious.