TENS units (Transcutaneous Electrical Nerve Stimulation) deliver low-voltage electrical pulses via adhesive electrode pads to block pain signals and stimulate endorphins for drug-free relief. They target back pain, arthritis, sciatica, and post-surgical discomfort using high-frequency (80-120 Hz) for acute pain gating or low-frequency (1-20 Hz) for chronic endorphin release. Portable battery-powered devices like Beurer EM59 combine TENS/EMS with optional heat; place pads 1-inch apart on clean skin near pain sites (e.g., lower back channels). Start low intensity for tingling (not pain), 20-30 min sessions 2-3x daily.