You may think as many do, hypnosis is a state of deep sleep.  No it’s not it’s trance like state, with heightened awareness with the client focussed on the therapists voice. It does involve the induction of a trance-like condition, and In this state, the conscious mind is rested and the subconscious mind is revealed.

The therapist is able to suggest ideas, concepts and lifestyle adaptations to the client, planting the seeds of positive suggestion which become firmly embedded to change thoughts feelings and behaviours. The practice of hypnotherapy promotes healing and positive development for clients.

Hypnotherapy aims to re-frame patterns of behaviour within the mind, enabling irrational thoughts feelings and behaviours, negative mind blocking and suppressed emotions to be overcome. As the mind is released from conscious control during the induction into a relaxed trance-like state of hypnosis, breathing alters becoming slower and deeper, it is observed the pulse rate drops and the metabolic rate falls.   It has been observed that similar changes along neuronal and hormonal pathways enable the sensation of pain to become less acute, and the awareness of unpleasant symptoms, such as nausea or indigestion, to be alleviated.

Does it work?

The use of hypnotherapy with cancer patients is another area being investigated. A meta-analysis of 116 studies showed very positive results of using hypnotherapy with cancer patients. Ninety-two percent showed a positive effect on depression; 93% showed a positive effect on physical well-being; 81% showed a positive effect on vomiting; and 92% showed a positive effect on pain.
Source: http://medical-dictionary.thefreedictionary.com/hypnotherapy

How does it work?

Hypnosis is thought to work by altering our state of consciousness. The conscious mind which controls us is inhibited by inducing a trance state, and the subconscious mind awakened and spoken to by the therapist.  The subconscious mind is a deeper state of mind and is more instinctive and intuitive than the conscious mind. It is this process of working with the unconscious that is the part which has to change for the client's behaviour and physical state of mind and body to alter.

If someone wants to overcome their fear of a phobia they may try a variety of methods using their conscious mind in an attempt to overcome their percieved fear or phobia. Yet they will still fail as long as their subconscious mind retains the fear and terror preventing them from succeeding. Progress can be made by reframing the subconscious mind so that deep-rooted values and beliefs are changed.

General Hypnotherapy Register
Complementary Natural Healthcare Council

Limitations to Hypnosis are:

Hypnotherapy shouldn't be used if you suffer from psychosis or certain types of personality disorder, as it could make these conditions worse.  If you have any type of mental health problem or serious illness, such as cancer, make sure the hypnotherapist is trained in working with your particular condition. Similarly, children should not be hypnotised by anyone who isn't trained to work with this age group. Source NHS Choices 2015