I'm not sure if I have the same thing, it sounds like it by the description, but mine is a TENS unit (transcutaneous electrical nerve stimulation). At any rate, if they are the same, I can't tolerate it for more than an hour. It feels great at first, then gets annoying and by the end of an hour starts to make it worse for me.
I've been guilty of grabbing the pad in a knee-jerk reaction when it slipped....you'd think by that zap I'd learn not to do it again...but sure enough......&*@#$ it happens every time.
I bet it equates to the shock-collar they use to train the dogs to stay in boundaries without the invisible fence!