I think that all these devices are working on the same principle as magnetic therapy, they increase the blood circulation which brings oxygen and nutrients to the targeted area.
T.E.N.S means = Transcutaneous/ Electrical/ Nerve /Stimulation

Transcutaneous means 'through the skin'. TENS machines deliver small electrical pulses to the body via electrodes placed on the skin. TENS machines are thought to affect the way pain signals are sent to the brain. Pain signals reach the brain via nerves and the spinal cord. If pain signals can be blocked then the brain will receive fewer signals from the source of the pain. We may then feel less pain.
When the machine is set on a high pulse rate (90-130 Hz) it triggers the 'pain gate' to close. This is thought to block a pain nerve pathway to the brain. This is the normal method of use.
When the machine is set on a low pulse rate (2-5 Hz) it stimulates the body to make its own pain easing chemicals called endorphins. These act a bit like morphine to block pain signals.
Brian