I used to suffer back pain, and went to a chiropractor regularly for it. He used an electrical stimulation device, possibly similar to what's being discussed. I definitely experienced improvement from the visits.
However, my wife and I committed to a real exercise regimen, and this has improved the situation so that I no longer see the chiropractor (and feel better than when I had). I suspect that the crunches and other abdominal exercises are what's relevant here.
-CWuestefeld
Two separate ways of decreasing pain have been raised in this thread: TENS unit usage to decrease pain by Mouser and subsequently exercise by CWuestefeld. Since pain control mechanisms vary greatly with these two approaches it's probably best to keep them conceptually separate.
Exercise is sometimes an extremely helpful aspect of pain management. In the case of the back, exercise strengthens the abdominal or stomach muscles. As these muscles increase in strength, they can serve a greater role in postural support. Pain then decreases because there is less stress on the spine.
In contrast, a TENS unit serves a strictly palliative role rather than attempting to modify the underlying pathophysiology. As described in Mouser's post, these units decrease pain by disrupting the "pain signal" being transmitted through the nerves. An additional or alternative approach used by some (particularly newer) devices, is to stimulate the release of the body's natural pain reduction substances (endorphins) into the muscles.
As CWuestefeld indicated, it's preferable to eliminate or decrease a medical problem rather than just treating the resulting symptoms. Sometimes this is not possible though or even potentially dangerous. When this is the case, treating the symptoms with a TENS (or any other approach) is all that's left.
Regarding the two approaches, signal disruption is the older one and it appears that an increasing number of devices are being designed to release endorphins or to do both. Consistent with this shift, an informal review of the information available on the internet appears to suggest that TENS units that stimulate endorphin release decrease pain more.
Ken