Hypnotized

In response to Chapter 11, I found the section titled "Waking Sleep" to be very interesting. "Hypnosis is described as a state of consciousness, dissimilar to either wakefulness or sleep, in which attention is withdrawn from the outside world and is concentrated on mental, sensory, and physiological experiences"(158).
Through hypnotism, a person can relive pain and allergies, control or relieve amnesia and deal with various other ailments. "Hypnosis can also produce a deeper contact with one's emotional life, resulting in some lifting of repressions and exposure of buried fears and conflicts"(158).
An individual can be greatly helped and healed through the use of hypnotism. But, as far as I understand, not everyone is able to be hypnotized. And what allows someone to be hypnotized? What is the difference between someone who can and someone who cannot?
I suggest, since hypnotism is considered an altered state, the "restricting valve" is tightened on those who cannot be hypnotized. Which relates to the opened valve of the creative personality, the shaman, and even the "classified" mentally ill. But there must be more than the "restricting valve" or limited degrees of restriction in determining one's ability to access the an altered state of consciousness. Because not everyone who is creative can be hypnotized. And, not everyone who is mentally ill is creative. Therefore, I would really like to learn more about the hypnotizing personality in relation to the altered states.