E-stim can be used for a variety of reasons. At low frequencies, e-stim can be used to temporarily “block” pain so that treatment or exercises can be performed.
This works by the same logic that rubbing your elbow after hitting it on something stops the pain. The brain is only wired to acknowledge one sensation at a time, and vibration signals travel along a faster path than pain signals.
At a higher frequency, e-stim can be used to fatigue muscles. This frequency is used when there is a chronic muscle spasm that is blocking motion and guarding against treatment. This works by forcing the muscle to use up all of its stored energy, leaving no energy to sustain a contraction.