The exact nature of the treatment is determined by the frequency of the pulse: high frequency, typically greater than 60Hz (60 pulses per second), attempts to override the pain signals being sent to the brain; low frequency, accompanied by high intensity, attempts to stimulate endorphin release.
In theory, TENS sounds like a good idea.
However, the effects are mild (at best) and very short-lived. The high frequency mode, for example, is not very good at overriding the signals being sent to the brain and the effects stop as soon as the current is turned off.
The low frequency mode is slightly more effective, but:
It does not produce many endorphins.
The endorphins that are released, are broken down by the body relatively quickly (using an enzyme called endorphinase); their effects are therefore short-lived.
The body develops a self-limiting tolerance to these chemicals over time, just as it does for any other drug (naturally occurring or otherwise), an effect known as down regulation.
At best, TENS will provide symptomatic relief from relatively mild back pain and only for short periods of time. It does not treat the causes of back pain.