Hypnotherapy is based on the idea that there are vast internal (unconscious) resources that are largely untapped and that can be utilised to make the changes the client wishes.  The client remains aware and in control at all times.  Only ideas or actions that are in accord with the internal standards and values of the client will be accepted.
The aim of hypnotherapy is to by-pass the conscious mind, which is always busy analysing, and to access the unconscious mind so that changes can be more readily accepted and acted upon.