TENS is thought to work in two ways:
By selectively stimulating certain 'non-pain' nerve fibres to send signals to the brain that block other nerve signals carrying pain messages
By stimulating the production of endorphins, natural pain-relieving hormones
The device is usually used for 15 to 20 minutes, several times a day, and is controlled by the user rather than a health professional. Pain relief may be rapid and last for days.
Unlike many pain-relieving drugs, TENS isn't addictive and seems to have few side-effects.