On a somewhat related subject, this explanation of the TENS concept is one of the best I've read. It came from a source that was very skeptical of its effectiveness, but I think this is really what happens when I use my device:

"The brain decides what hurts based on many sources of information. TENS messes with that system. It blasts the nervous system with static, sensory white noise. By stimulating nerves in this strange way, TENS doses the brain with unusual sensory information, which either “drowns out” information about tissue damage, and/or simply distracts the brain and forces re-evaluation."

Brains are weird things, though, and I don't think mind is "normal." I have a highly logical/technical mind. Despite my best efforts, I can't meditate, nor can I be subject to the effects of hypnosis (as best I know). Falling asleep has always been problematic as my brain won't allow it until it's "done" for the day...no matter how tired I am. For whatever reason, my conscious mind will not readily relinquish control.

With TENS, however, it seems to cave to the physical stimulus. I suspect I am MORE sensitive to it for whatever bizarre reason, and it works for me. It's not a cure-all, but it definitely helps.