electrifying

i am looking to play with a tens unit but i am unsure what the rules and warnings are for this...some people say you cant use it at all above the waist yet some people say you can...i have the unit that i got for an ankle injury years ago but i have never used it for any kind of play...i love breast play and i am hoping to find some advice on doing it..but i don't want to do any serious damage to myself so i am looking for others input...luckily i have complete trust in my old man who will be the one controlling the session...so i feel 100% safe in that sense...i just want to have an idea of what the guidelines for this kind of play should be...also i am wondering what kind of sensations the tens unit will provide..so as always any comments or suggestions will be greatly appreciated