--especialy about starting at low settings, both so that one doesn't "habituate" away from any pain-blocking effect too rapidly, and so that muscle spasms are not produced.

I have a nice programmable four electrode portable TENS (with belt clip) that was originally prescribed through my physical therpaist for the meralgia parasthetica in my right thigh (which has been flaring recently, possibly with all the driving I've been doing), but I've used it over many parts of my body (though not the face and chest). The intensity levels for each electrode range from 1 to 36 and there are twelve pre-programmed patterns that can be chosen, ranging from pulses of various durations to continuous current. I generally use intensities ranging from around 5 to 12, though as the battery loses charge (they are rechargable) I have to ramp it up somewhat. In general, the fleshier the area (say, a thigh compared to a forearm), the higher the setting needs to be to get the same effect.

It does have a good effect for me as regards pain blockage, at least as long as it is on. TENS units take advantage of the nerve conduction fact that pain is the sensation that moves most slowly up and down the carrying fibers; overlaying another sensation, such as electrical vibration, over the pain one tends to block it from perception.

Electrode placement is a big experiment, especially as none of us have exactly the same pain or exactly the same neuroanatomy. Whenever I use it, I typically move the electrodes around, millimeter by millimeter, a number of times until I find an optimal spot.