To understand how and why hypnosis works you first need to understand that we have two 'minds': the Conscious and the Unconscious.
Your beliefs about yourself are stored in your unconscious- eg:
Its the beliefs we hold about our musical ability that we need to change. Here's how:
When the brain becomes very relaxed it enters 'Theta Brain-Wave' state called Trance. In this state the 'critical' function of the conscious mind is disabled, and suggestions are accepted by the unconscious. Our unconscious mind has an elaborate 'alarm system' and it will not accept, or act upon, an un-ethical suggestion. You'll find a more thorough explanation of the mysteries of Hypnosis on the new podcast.