@trojanlarge - E-Stim is one of the many types of electric play (which can also include, but is not limited to: violet wand, abdominal muscle stimulator, electric fly swatter, electric dog collar, stun gun, electric cattle prods). TENS Unit, Folsom, Rimba and PES are collectively e-stim from different manufacturers.
E-Stim is electrical sensation play, which can range from sensual to painful, depending on the person and the settings of the equipment. You can do long or short pulses, and light (nice rhythmic pulsation) to medium (stronger, with a slight "bite") to intense/heavy (strong, with a stinging pain) sensations. Some units allow it to cycle in different patterns (i.e., "yeah, yeah, yeah, OW!"). Basically, if you place the contact pads in the right areas, you create a series of muscular contractions/spasms at specific intervals which can induce one or more orgasms if you hit the right spot(s).
The way it works is that electrical current passes between two contact points, which can be on one attachment or two attachments. The unit providing the electricity (the "e-" in e-stim) will either be a 9-volt battery or a wall plug (not both). As previously mentioned, never run an electrical current across the chest or heart area, and never use an electrical toy near or in conjunction with water. Know the medical condition of your play partner, and -- sorry -- if you have a known heart condition (pacemaker), "No electricity for you." (in my best Soup Nazi voice)
For a guy, you might enjoy doing a catheter on one lead, and a butt plug on the other lead for your prostate. Your issue might be that you're not completing the cycle with a second lead to create the electrical current that runs from Point A to Point B . . .?
I attached a nice link that I found that has a lot of FAQs about E-Stim