Transcutaneous Electrical Nerve Stimulation (TENS) involves a mild electrical current being passed through the skin, into the nerve fibres that lie just beneath it. The current is delivered in a series of pulses by a pair of small electrodes, placed around the site of pain.

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.

Endorphins are naturally occurring opioids, the class of drug to which heroin and morphine belong. They are produced by the body in response to various stimuli (the term end-orphin literally means endogenous morphine, or morphine that occurs from within) and, just like morphine, they change our perception of pain (making it appear less noxious).