Very simple. Nerves are like small wires. You dump voltage down them they aren't designed to carry and you do damage period. Any electrical component that receives shocks has a shorter life. Is the body not bio-electrical-mechanical system?

By the way "Pica" as reason to torture? Anyone who eats ice can be diagnosed with that lame duck disordered.

No one should receive electrical shocks unless they can give informed consent. Unable to explain how it works what happens to the body, then you aren't informed sufficiently. No one truly moral and ethical would decide to inflict pain on another for their own good. Any person who believes torture is acceptable need to be inflicted with their cure until they learn.