EMS sends mild electrical pulses through the skin to help stimulate injured muscles or manipulate nerves to reduce pain. EMS uses electrical pulses to mimic the action of signals coming from neurons (cells in your nervous system). These mild electrical currents target either muscles or nerves.
E-stim uses small electrodes placed on the skin. The electrodes are small, sticky pads that should come off with little discomfort at the end of the session. Steady streams of electrical pulses are delivered through the wires from the e-stim unit. For muscular stimulation, the pulses will reach the muscles, signaling them to contract. Pulses aimed at the nervous system block the transmission of pain signals from reaching the spinal cord and brain. The pulses also stimulate the body to produce more natural pain-relieving chemicals called endorphins.
Info obtained from https://www.healthline.com/health/pain-relief/e-stim#how-it-works