http://www.tens-store.com/ems/tensunit.html
Pain relief with TENS (Transcutaneous Nerve Stimulation) is produced through high frequency electrical stimulation of the nerve which disrupts the pain signal so that the pain is no longer felt.
I'm not sure if the "prescription required" lark is just a put-on, in the UK, it's not essential, but it lets you get them with no VAT.
The EMS units - most basic of which is the cheap "ab toner" belt tend to operate with multiple and variable patterns - if they offer a simple pattern, they may be suitable for tens use (one WAS sold as dual function), but the mediacal tens units usually only have variable rate and intensity - when you've hit the spot, you don't want it changing rate or intensity.
A friend was loaned a TENS unit from the hospital - they were originally planning to OPERATE (disc problem, can't remember what) but it recovered itself after TENS relieved the tension. For TENS to really work, I supect you DO need to be shown the correct way to use it.
PS. I had to repair my dad's TENS unit, and despite their vain attempt at blocking out the chip identity, I recognized it as a "555 timer" (a VERY cheap chip used for oscillators/timers) driving a small transformer, the classic "shocking coil" configuration - very expensive for what it is!