In a hypnotic state (trance0 research has shown that the critical faculty of the mind which is best described as the filter between the conscious and unconscious mind can be bypassed. Therefore make it much easier to work with the unconscious mind which is the key to creating change.
In a conscious state this critical faculty can be likened to a guard standing over our unconscious mind to filter and protect what goes in.
Our , beliefs, memories, experiences, behaviours, habits etc are programmed into our unconscious mind. You can decide to change with the conscious mind however the unconscious mind creates the change AND for most change to be permanent, it must be impressed onto the unconscious mind where it becomes an unconscious habit (happens automatically without thought).
Just think about behaviours you have that you don't even think about....these are unconscious habits. How you get out of bed in the morning, what is the first thing you do....which shoe you put on first, these are all automatic unconscious habits. Unconscious meaning we are not aware we are even doing them.
In a hypnotic state or some people use the word trance - a hypnotist can work directly with the unconscious mind to recode memories, associations, experiences etc to change beliefs, habits and behaviours that often keep us stuck or preventing us changing.
Therapy carried out in a hypnotic state/trance can often create change quickly and permanently. Each hypnotist/hypnotherapist uses their own techniques to bring about change. There is no specific set of techniques to use for a particular issue.
Although some techniques bring about such good results quickly that they are used often by many hypnotherapists.
To summarise, hypnosis works because the hypnotic state allows us to bypass the critical faculty and work directly with the unconscious mind. However, the correct therapy for the client needs to be applied whilst they are in a hypnotic state for a change to take place.