Non-Instrumental Movement Inhibition (NIMI) is an aspect of body language when a person stops fidgeting because they are interested in what they are watching.
As such, it is a psychological phenomenon and a form of embodied behavior, where gestures and body movements reflect the thoughts and emotions in a person's mind.
Non-instrumental (unnecessary) movements include fidgeting, scratching, postural micromovements (e.g. sitting forward in a chair), certain emotional expressions (e.g. shrugging), and even breathing.
[4] The original observation that, in a seated audience, interest is associated with diminished fidgeting, and that boredom doubles the amount of human movement, was made by Francis Galton in 1885.
[9] They also demonstrated that, during individual human computer interaction in instrumentally identical reading comprehension tasks, interest itself was sufficient to diminish movement.