From: http://www.hypnosiseducationcenter.com/ideomotor-action.html
Ideomotor Action
Human thought manifests itself in two forms. We can think using visual images or by using words (language). Language consists of sound symbols used as a means of communication between two individuals. Objects in the exterior world are designated by sound symbols that permits them to be envisioned even in their absents. However, words become detached from the objects they symbolize and take on an independent life. They no longer are only a means of communication, but become an instrument of thought. Words become an internal language, which is no longer speech, since it is not expressed in sound, but a method of thinking.
No matter which method we use for thinking, the end result is a change in the musculature of our body. If you think of performing some act, the muscles of your body responsible for carrying out that act are enervated whether you actually carry out the act or not. If you think about buttoning your shirt, or describe how to button a shirt to someone else, the muscles of your body that would be used to perform the action are energized and in an aborted way carry out the act. This is why the Semantic-Relaxation Exercise works. If you suggest to yourself or imagine that a muscle or groups of muscles are relaxing, they actually respond to some small degree. Through repetition the response becomes greater and more generalize. I don't expect you to believe this. I intend to let you perform a little experiment and prove it to yourself. For the experiment you will need a piece of string or thread and a small weight of some kind (i.e., heavy button, bead about 3/4" in diameter, etc.).
Attach your weight (button etc.) to the string as illustrated at the right. What we are creating is just a pendulum, a small bob hanging to a thread or string. Almost any small object will do. The dimensions given are not critical. If you use a button it should be about 3/8 to ½ inch in diameter. The color is unimportant. What you will have created, if you decide to make the object above, is called Chevreul's pendulum. This experiment can be performed while you are in any comfortable position. Just hold the end of the string so that the weighted end can swing freely. One way to do it is to sit at a table. Rest your left forearm in your lap or along the edge of the table. Rest your right elbow with your right arm lifted almost vertically, a little forward of your left arm. Hold the end of the string between your finger and thumb. Position yourself so that you are comfortable. If you are sitting as described, the pendulum should hang in front of the center of your body (median plane).
The bob should be about ½ " from the top of the table. Actually none of these instructions are crucial, if you approximate them the experiment is virtually certain to succeed. After you have assumed your position, the bob will probably be swinging freely in some direction. With your left hand bring it to a stand still (if you are left handed, reverse hands). Put your hand back into position. If the pendulum is not perfectly still don't worry about it, its not that important. Now fix your gaze on the bob and imagine that it is the pendulum on a grandfather's clock. Imagine that it is swinging from side-to-side just like a pendulum on a clock. The more you can see the pendulum swinging from side to side in your imagination, the greater the response will be. Follow with your eyes an imaginary pendulum on a clock moving from side to side. Try not to think about anything else and do not try to analyze what is happening. If you followed the above instructions, the pendulum will do exactly what you imagine.
Once you have the pendulum swinging from side to side, without stopping it, imagine that the bob is sitting on the edge of a phonograph record and the record is going around and around. As you imagine the bob going around on the outer edge of the disk, you will find that the side to side motion changes into a rotary motion. Once the bob starts going in circles, imagine that the record is spinning faster and faster. The bob will begin to make a larger circle and pick up speed. Some individuals when first trying this experiment tend to tense their muscles. This should not be done and will only serve to lessen the response. Try to be relaxed when you try this experiment. The only people that this experiment will not work for are those individuals that "know" before they start, it will not work. However, if they are thinking, "this will not work, the bob will not move." The experiment is working for them, they are getting exactly what they are thinking, "the bob will not move" and it will not.
The Chevreul pendulum experiment is a demonstration of the phenomenon of ideomotor action. That is, the tendency of thoughts or ideas to be automatically translated, reflex like, into specific patterns of muscular activity. Probably many hypnotic suggestions act purely through ideomotor action. As stimuli they trigger corresponding thoughts and images that act as cue-producing responses, evoking actual motor responses. The repeated elicitation of an ideomotor response increases the intensity of the response.
Ideomotor actions tend to be weak responses, particularly in their initial phrase. If there is present another strong muscular action or tension involving the same muscles it will tend to mask or block the weak pattern induced by thoughts. The probabilities are that you will succeed with this experiment as very few people fail to get a response. If you do not get any results, do not give up. Try again on several consecutive days or try using a lighter or heaver bob. You might try a different length of string. In general, a longer string will give a greater response.