One thesis Braude examines in First Personal Plural is that MPD is a form of hypnosis.
Hypnosis allows remarkable dissociations, particularly with respect to sensations of pain.
In the mid-1800's many operations were performed using only hypnosis as the anesthetic, with apparently successful results. If he asked a hypnotized subject not to feel a pin-prick, then the subject would report feeling nothing. But if he asked whether no part of the subject felt pain, then the subject would answer in the negative. The existence of the hidden observer raises the question of exactly what is happening in hypnosis. The part that retains control of our bodies, including the vocal system, simply ignores or cuts off the part that feels pain or the part that suffered some trauma.