This is partly explained in the literature as being the result of ATP synthesis, a known process that occurs in the muscular tissues. The long term effect is increased strength, bone, tendon and muscle growth.
There are a vast number of tissue types in the body; stimulating them all, even slightly, seems to result in a reliable turn around for a great many people.
Other noted effects include a variety of bio-active electromotive forces. The endergonic reactions brought about by frequency and polarity result in microbial electrocution and detoxification. These may also include many other effects, such as diathermy, electrolysis, nerve adaptation and electroporation to a much lesser degree.