TENS works by passing harmless electrical signals into the body from its pads. This relieves pain in two ways:
Firstly, "high frequency" continuous, mild, electrical activity may block the pain signal travelling to the brain. Brain cells perceive pain.
Secondly, TENS stimulates the body release its own natural pain-control mechanism. "Low frequency" or short bursts of mild, electrical activity may cause the body to release its own pain easers, called beta endorphins.