Hi Nics! E-stim is the stimulation of nerves to create sensation with mild current. It's effect can be widely varied based on the signal type and the kind of electrodes used. It can feel like a soft tingle, a strong vibration, thumping pulses, gentle waves or quick rapid strokes. With vaginal electrodes, it can actually cause muscle contractions that will thrust a probe in and out of you. A lot of people are apprehensive because "electricity" is involved, but most people only associate what comes out of the wall with the thought. E-stim is very safe and insanely erotic!
There are a couple of kinds of e-stim. The most basic is a T.E.N.S. unit. Its generally designed for medical applications and physical therapy. These units get you a sens of what stim will feel like, but they are more like one or two keys on a piano, they make sound, but you can't compose a masterpiece with them.
The next kind of units are made for play boxes. These include the PES Electro (http://www.peselectro.com) box, Folsom Boxes, and the Cadillac of E-Stim the Erostek ET-312 (http://www.erostek.com). These boxes range from about $200-$500 dollars without electrodes.
The ET-312 is so widely regarded because it has a ton of built in programs that really vary the feelings and sensations you get. For instance, there is a mode called Stroke that will move a vibrating feeling between two electrodes. If you placed at the base of the penis and one at the glans, the feeling will move back and forth between those electrodes and you can control the speed and intensity of the stroking.
Another mode is called Orgasm. This mode slowly builds in speed over 3-4 minutes gently teasing you towards orgasm. The waves get closer and closer together and eventually pulse at the .8 second interval of orgasm. This makes you cum and feels amazing.
The best part of electro stim is that it is can be totally hands free allowing you to lay back (or be restrained) until you cum again, and again. Sometimes the feelings are so intense I just go rigid with pleasure.
Keep in mind a lot of the video you see on the internet of electro stim is either fake, or done with a BDSM mindset. It's not an accurate picture of what e-stim is like. There are a lot of different methods to try estim, if you shoot me a note I'd be happy to help introduce you.
-Alan