Darren Brown gets someones wallet just by asking

For those who wonder how he did it, we can look to American psychiatrist Milton Erickson who writes on the specific technique used by Darren Brown.

The Handshake Induction

Confusion is the basis of Erickson's famous hypnotic handshake. Many actions are learned and operate as a single "chunk" of behavior: shaking hands and tying shoelaces being two classic examples. If the behavior is diverted or frozen midway, the person literally has no mental space for this - he is stopped in the middle of unconsciously executing a behavior that hasn't got a "middle". The mind responds by suspending itself in trance until either something happens to give a new direction, or it "snaps out". A skilled hypnotist can often use that momentary confusion and suspension of normal processes to induce trance quickly and easily.

By interrupting the pattern of a 'normal' handshake in some way, the hypnotist causes the subject to wonder what is going on. If the handshake continues to develop in a way which is out-of-keeping with expectations, a simple, non-verbal trance is created, which may then be reinforced or utilized by the hypnotist. All these responses happen naturally and automatically without telling the subject to consciously focus on an idea.

The various descriptions of Erickson's hypnotic handshake, including his own very detailed accounts, indicate that a certain amount of improvisation is involved, and that watching and acting upon the subject's responses is key to a successful outcome. The most important thing is that the 'normal' handshake is subverted in such a way to cause puzzlement, which may then be built upon.

* Initiation: When I begin by shaking hands, I do so normally. The "hypnotic touch" then begins when I let loose. The letting loose becomes transformed from a firm grip into a gentle touch by the thumb, a lingering drawing away of the little finger, a faint brushing of the subject's hand with the middle finger - just enough vague sensation to attract the attention. As the subject gives attention to the touch of your thumb, you shift to a touch with your little finger. As your subject's attention follows that, you shift to a touch with your middle finger and then again to the thumb.
* This arousal of attention is merely an arousal without constituting a stimulus for a response.
* The subject's withdrawal from the handshake is arrested by this attention arousal, which establishes a waiting set, and expectancy.
* Then almost, but not quite simultaneously (to ensure separate neural recognition), you touch the undersurface of the hand (wrist) so gently that it barely suggests an upward push. This is followed by a similar utterly slight downward touch, and then I sever contact so gently that the subject does not know exactly when - and the subject's hand is left going neither up nor down, but cataleptic.
* Termination: If you don't want your subject to know what you are doing, you simply distract their attention, usually by some appropriate remark, and casually terminate. Sometimes they remark, "What did you say? I got absentminded there for a moment and wasn't paying attention to anything." This is slightly distressing to the subjects and indicative of the fact that their attention was so focused and fixated on the peculiar hand stimuli that they were momentarily entranced so they did not hear what was said.
* Utilisation: Any utilisation leads to increasing trance depth. All utilisation should proceed as a continuation of extension of the initial procedure. Much can be done nonverbally; for example, if any subjects are just looking blankly at me, I may slowly shift my gaze downward, causing them to look at their hand, which I touch and say "look at this spot.". This intensifies the trance state. Then, whether the subjects are looking at you or at their hand or just staring blankly, you can use your left hand to touch their elevated right hand from above or the side - so long as you merely give the suggestion of downward movement. Occasionally a downward nudge or push is required. If a strong push or nudge is required, check for anaesthesia.

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Richard Bandler was a keen proponent of the handshake induction, and developed his own variant, which is commonly taught in NLP workshops.

Any habitual pattern which is interrupted unexpectedly will cause sudden and light trance. The handshake is a particularly good pattern to interrupt because the formality of a handshake is a widely understood set of social rules. Since everyone knows that it would be impolite to comment on the quality of a handshake, regardless of how strange it may be, the subject is obliged to embark on an inner search (known as a transderivational search, a universal and compelling type of trance) to identify the meaning or purpose of the subverted pattern.