E-stim makes use of electrical pulses to act as the signals carried by neurons. These currents are really mild and mostly focus on the conditions you have on your muscles or nerves.
In the case of muscle injury, E-stim therapy sends signals to the area in the muscles that have to be treated to make them contract and repeat this process of contraction and relaxation over and over. It will thus improve the blood flow and help repair the injury.
Also, continuous contracting and relaxing through this therapy "train" muscles to respond to the natural signals from the body to contract. This treatment is highly beneficial for stroke survivors who need to relearn basic motor functions.