Hypnosis works by relaxing you into a state where your brain shifts its activity. In fact, on a brain scan, a person in hypnosis shows the brainwave activity as we experience in the early years of our lives when our minds were able to sponge up information and immediately integrate it. This same accelerated learning occurs in hypnosis, bypassing your conscious mind so that your unconscious beliefs and perspectives change quickly and naturally (and it was in your unconscious mind that old limitations and worries were causing you to experience the challenges and discomforts that you're here to change!)
The hypnotist causes this trance formation by using subtle hypnotic commands to put you into a mild trance, a state where your conscious mind is largely in-active, direct access to your subconscious mind can be gained, and positive suggestions can be planted.
Because of this there is no conscious resistance from you, and so changes can be made to long-held belief systems. Often those belief systems were not beneficial, yet they may have been so familiar to us that we would hang onto them and become “logically” defensive of them. For example you might have had a low self image and want to change it, but your fear of the unknown, and relative level of “comfort” at thinking in this way made you resist against change.
However with the power of hypnosis, this logical, conscious thinking function is bypassed and changes are made from within - positive changes which alter your beliefs, how you feel about yourself, and in this example, ultimately your levels of self esteem improve.