Hypnosis allows the therapist to communicate with your subconscious mind and change the patterns that create behaviours that we no longer want.
It could be said that most of the psychological problems we suffer are in fact where the subconscious is trying to protect us. It maybe that we have been hurt (emotionally or physically) in the past and now the mind is seeing almost everything as a threat and so goes in to overdrive causing anxiety and even panic attacks. Many peoples weight issues are caused by the brain hanging on to old issues from the past where certain foods were used as treats or comfort so that now when you are upset or hurt you immediately look for the comfort food. Hypnotherapy helps deal with the issues you have collected over the years.