One of the main contributing factors affecting muscle strength is the ability to recruit fibers for a particular muscle group. NMES can stimulate and recruit more fibers of the muscle group that lacks muscle fiber recruitment.
The electrical impulses trigger muscle contractions. Through repeated practice with the assistance of NMES, your body becomes better at finding its way to recruit that muscle. With increased muscle fiber recruitment, it regenerates the axon in the nerve, allowing you to recruit more muscle fibers.
This is helpful in the initial phase of physiotherapy, especially if you have difficulty contracting your muscles. It is very useful in re-educating the muscles, especially after post-op, accelerating the learning process of activating your muscles.