I'm not a super expert, but a lot of it is common sense. Don't play if you don't understand what's going on, first off, and don't take this post as a concise course on what to do. And use electrical sources specifically designed for use on living creatures - this means, to me, homemade is completely NOT ok. There are three main types of electrosex type devices that you might run into in common use: Violet wands, TENS style units, and shocking units like a cattle prod or electric dog shock collar.
Violet wands are safe to use anywhere on the body, because (as I understand it) they're about surface shocks (the glass is electrified and is a poor conductor, so the electrity will take the opportunity to leap to your skin, which is a better conductor, but then that's it, because the source is no longer "feeding" the electricity) - except, you know, maybe not the eyelids, and not internally unless the attachment is very specifically designed for that. I could be off there, however, I haven't spent much time figuing out how violet wands work cause I don't have one. But they are for sure safe to use anywhere...and often look very dramatic. And feel heavenly...
Cattle prods are capable of delivering a shock potent enough to muck with your heart, so they're only safe to use below the waist (and, oh, they're SO DELICIOUS). The zap with a cattle prod or something similar happens when both sides of the unit come into contact with the skin - the circuit thus being a flow from one electrode, through the skin, to the other electrode.
TENS style units - like the nexus iStim, Erostek PES boxes, or regular therapeutic TENS units, are designed to zap very superficially - much shallower than a cattle prod would - and are safe to use (in my opinion) almost anywhere. You know those late night TV ads from "Dr. Ho"? That's a TENS unit. Dr. Ho sticks them all over chests...and even on heads! The thing to remember with a TENS unit is that no matter what the accessory - be they tens pads, or clamps, or devious devices, keep in mind the way you're completing the circuit - out one electrode, through the body, and into the other. For obvious reasons, you never want this circuit to include the heart, thus the "rule" with estim is "never across the chest cavity". Now this can get pretty complicated if you have a TENS style unit with two ports. Yes, this means go ahead and set up the pads (/accessories) in two different areas...but you have to make sure that there's no cross port currents that could go across the chest cavity. What I mean by that is you cant's set up one pair of tens pads on the front of your chest, and another on your back. I think the chances of something actually happening if you did this would be pretty slim, as these devices are designed to give very shallow shocks - but hey, safety first. Also, keep in mind electricity will always take the path of least resistance. And backing off to take an even safer stance, if you don't understand and insist on trying to play anyways, keep it below the waist.
It's seriously a world of fun
I love playing with estim.
A lot of play parties will have the blanket rule of "No electrical play above the waist" some will include "except violet wands". But the real "rule" is "don't direct electricity across your heart".
DBD, what did you see, exactly? At the parties I've been to, people that are playing with estim toys are more than happy to explain how they're using them and how it makes sense, so I would recommend asking in a curious way - I know I love nerding it up and explaining what my mad scientist gizmos do. Just wait till their scene is over, of course.