Your therapist guides you into a relaxed state of mind called "trance" in which it's easier to interact with your subconcious. In this state you can work on past experiences or beliefs about yourself and your stutter. Basically you can reprogramm your brain to some degree. Important to note: you stay awake throughout the whole thing and you are in controll of your actions. It's helped me be more comfortable with my stutter and I made a few important realisations. Hypnotherapy can be over in a few sessions and I don't think you will see much improvement after about 10 sessions. I personally had about 5 sessions so far and will probably do 1 or 2 more. ​ All this is my personal experience, I'm sure you can find more generall information if you look it up.