One of the options that your physical therapist might use to relieve your back pain is the use of a TENS machine.  TENS stands for Transcutaneous Electrical Nerve Stimulation.  The theory behind TENS machines is that the mild electrical stimulation that the unit supplies overrides the pain signal that you receive from your brain.  As this pain signal is blocked, you receive relief from the pain.

A TENS unit works by placing two electrodes at different points of your body.  The TENS unit will then send a mild electrical current between those two electrodes stimulating the nerves and/or muscles.  The unit will allow you to control the amplitude, frequencies and pulse widths.  I know it sounds scary, but these units really work.   Most physical therapists will have larger TENS units to treat you with, but there are also smaller TENS units that you can purchase and wear during the day to heal you deal with the pain.  These units are usually about the size of a pack of cigarettes and are powered by 16 volt batteries.

I own and use a TENS unit on occasion and have found it very helpful in dealing with my lower back pain.