TENS means Transcutaneous Electrical Nerve Stimulator. It is commonly used in Physical Therapy depts as a modality to treat denervated muscles like those in stroke or spinal injury patients. These are the ones you see with pads connected to the muscles to make it twitch or on higher frequency, to relieve pain. However, this machine should only be operated by a licenced physiotherapist with the supervision of a doctor. Untrained use of TENS might result to electrocution accidents.