Ideo Motor Responses (IMRs) are a frequently used tool of analytical hypnotism ('analytical' being where one wants to find out what's going on that drives a patient to a particular pattern of behaviour). The essence of IMRs is to get the subconscious mind (subc) to communicate directly without interference from the patient's conscious mind. This is achieved by asking the subc to take control of a finger or thumb of its choice which it will, thereafter, lift to signal a 'yes' answer, and when it has done so, raise it up. Once you have a 'yes' digit, you repeat the exercise to get a 'no' digit.
In training, practitioners are often taught to direct the 'yes' to the first finger of the dominant hand. Thus, with a right-handed person one would tell their subconscious to take control of the first finger of the right hand, with which it would be answering 'yes,' and raise it when it had control. The 'no' finger is then directed to the first finger of the other hand.
Personally, I don't see any merit in being this directive and prefer to let the patient's subc pick its own digits. Often it will use two fingers from the same hand. Sometimes it will move the patients feet instead. I think you get better communication, faster, if you let the subc express itself via whichever appendages it prefers.
All of which will sound pretty bizarre to anyone who hasn't witnessed IMRs in action Which you can do below. When a patient is sat on the couch watching their hands dancing about completely beyond their conscious control, however, the patient at least knows they are hypnotized.