During hypnosis you remain in full control. You would be able to react if there were an emergency situation. You can move, talk, listen and despite any preconceptions to the contrary, could never be made to do anything that you did not want to do. It is not possible to be hypnotised against your will and even when in hypnosis, a person can still reject any suggestion.
Simply put, when used appropriately in a clinical setting, hypnosis is not a method of one person controlling another. It is instead a state of co-operation, where the client and therapist can work together to achieve positive and beneficial change.