Freely printed from: https://learn.nimja.com/techniques/ideomotor_response
It is the automatic movement of the body (motor) by the mind or an idea (ideo).
These kinds of responses are often used in suggestibility tests but can also be used explicitly to communicate with the unconscious. The movement is often irregular and not smooth, which is a good indication that it's not the conscious mind moving.
A simple technique where you set up the subject to move their fingers in a yes/no response instead of answering verbally. It allows the subject to answer more (comfortably) with the unconscious.
This is usually done in 2 steps, with framing and testing:
Some people answer yes by moving the index finger on their right hand a large amount.
Move their index finger manually almost as far as it can go.
While others answer yes by moving the index finger on their right hand a small amount.
Move it manually about 75% of the way - So you always get a very visible response.
And of course repeat these instructions for the other hand.
Does your unconscious mind understand this?
Make sure the finger of the right hand moves significantly.
Now, please show me the "no" signal.
Watch for the signal.
Make sure the second line is not a question. If you ask if they can give you the no signal, the answer can be yes (they can).
You can use this technique on any body part in theory. Fingers are often chosen because they require very little effort to move. And with some hypnotic reinforcement you can make certain behavior present while someone is awake. Usually these suggestions fade like any others over time.