TENS, what is it?

Transcutaneous Electronic Nerve Stimulation, or TENS is being used in birth. They are most often used for back pain which doesn't respond well to other pain relief modalities. 

In Europe they are popular, but are becoming more so in the USA and the birth 
world. Quite intriguing to me, so in June, I shall take a class to learn all about TENS. Maybe I will have a new tool in my doula bag for my fall births!!

Have you used a TENS in birth or for other pain that doesn't respond well to the usual painkillers? What was your experience? Would you like to have a TENS available at your birth?