While in traditional training, the muscles are controlled by electrical impulses from the brain that cause contraction and thus a movement, in the EMS (Electro-Myo-Stimulation) muscles receive an external simulation from the electrodes.
The muscle is indifferent to the origin of electrical stimuli (brain or electrodes). Stimuli cause muscle contraction.
The muscle is a good conductor of electrical stimuli. The excitation of each muscle fiber depends on a sufficient intensity of the current (threshold of activity).
If this threshold is exceeded, the muscular fiber responds by a total contraction (law of all or nothing).
From a certain intensity, all the fibers are activated. A further increase in intensity has no effect.
So the machine only uses low currents (on a small scale micro amperes) not harmful to the human body.