Modern Hypnotherapy is a form of short-term psychotherapy that uses trance. Trance is a state of deep relaxation, whereby attention is focused on the inner world. It's a way to connect to deeper layers of the Self, and accedes subconscious processes (all knowledge, past experiences we are no longer aware of). In fact, trance is a natural phenomenon that everyone knows and sometimes experiences on a daily basis. Daydreaming and meditation are examples of it, as it is being absorbed in a good movie or book. You are then so focused that the outside world is temporary in the background.
Modern hypnotherapy has nothing to do with classical hypnosis or stage hypnosis where you lose control and cannot remember anything afterwards. Stage hypnosis is not a therapy.
During a session of modern hypnotherapy you are awake and focused, you experience what is going on in you, you are not out of control and you remember everything.