The main thing with stage hypnosis is the expectation and anticipation of the audience. Possibly the most important factor in achieving instantaneous trance is the prestige of the operator, and his skill to control the audience. The greater prestige the operator has, the greater the belief of the masses in his power and influence. This is why all performing hypnotists say they are "The World's Number One" and so on and so forth. For me, just having the title "Doctor" added prestige.


Let us imagine that a hypnotist is performing in your town or city. Posters are displayed advertising this hypnotist to be "The World's Number One" and other exaggerated claims such as "Master Of The Mind". In the eyes of the general public who don't understand mind and crowd control, the hypnotist is a person with amazingly mysterious powers and skills. The stage hypnotist uses the ignorance of the masses to his advantage.
People are drawn to see the show with preconceived ideas. Their own susceptibility to suggestion already heightened, they are already in a level of trance. Speed hypnosis, or instantaneous hypnosis is then fairly easy to achieve by a skilled operator.
All stage hypnotists and hypnotherapists know about post-hypnotic suggestions. The pre-hypnotic suggestion is less widely recognized or mentioned in books on therapy, yet it is highly important, and that cannot be stressed enough. The pre-hypnotic suggestion is vital in both areas of hypnosis. It goes back to the old saying, "As a man thinketh so is he". What you believe will be true for you, especially when trance is induced. Therefore if you think that after having experienced hypnosis you will have a headache, you will most likely have one. If you think that it might drive you insane, then it could. If you think it will be a good laugh, then it will be. But remember, there is no general rule of thumb!
With a stage setting many people are drawn to see the show as if by some compelling inner force. They are already controlled by their own fears and anticipation. Some of the audience are already on the verge of, or already in, a level of trance. Moods and feelings are contagious. The more packed the audience the more likelyHitler.  Hitler studied hypnosis and crowd control. He had a fascination around hypnotism and mysticism. you have a crowd. A large gathering of people does not necessarily make a crowd. A crowd is when the gathering of people are all experiencing similar thoughts and emotions, and this creates an element of collective unconsciousness. Man is a gregarious animal, and his mind can and does lose its individuality and personality when in a crowd setting. This is of course how mobs and riots occur. This is how a skilled politician can sway a crowd. This is how Hitler hypnotized a whole nation. A skilled stage hypnotist will use this to his advantage.
To turn a gathering of people into a crowd you need eloquence. You need to have everybody's attention, and as you talk to them and they listen, by suggesting what they will or will not experience, by adding strong emotional content, with repetition, with their minds collectively focused on a single pointed idea, that idea being trance, you already have most of the crowd in trance, and they don't even know it!
In some ways it could be described as a form of telepathy. There are numerous different types of telepathy. Most people when they hear the word telepathy think of mental telepathy. Probably the most common form of telepathy is emotional.
With a good crowd you have a very strong form of emotional telepathy. This of course can work for you, or against you if you don't understand it. It is the gut feeling, literally coming from the solar plexus region.
It is an invisible energy which not only do we feel, but we also transmit and receive. It is as if the solar plexus is the main emotional radio of the body. When we are close together as with a crowd these feelings transmit more easily from aura to aura, from person to person. This is why there is likely to be total panic when a fire occurs in a public building like a nightclub or cinema. A skilled operator can sense this, whether he is consciously aware of it or not, and can use it to his advantage. When the call for volunteers arises in a hypnotic show certain individuals just can't help but put themselves forward, moved by an invisible compelling force. That is to say, the positive pre-hypnotic suggestions of the hypnotist have already been made, and they come forward as the post-hypnotic suggestion is made calling for volunteers. The fact is that they are already being controlled because they are already in an element of trance. Their trance may have started when they first heard about or saw a poster advertising the event. It may have occurred only minutes beforehand. But it has already occurred.


So to summarize how stage hypnosis works, it is mass hypnosis. There is a level of collective unconsciousness, which is generally emotionally oriented through apprehension, based on fear. A gathering of people is transformed into a crowd. A crowd is dominated by suggestions of which it is unconscious.