It is generally considered to be an unconscious activity that the conscious brain has no true control over, but in various hypnotic theories - this statement may be taken to be actual hetero suggestion or role-playing to the contoller or therapist.
If you were to be asked to imagine doing up your shoelaces as vividly as possible, your brain would consciously fixate on the task and work through it as vividly and as logically as possible. The theory of IMR would imply that your muscular memory associated with your hands, would then attempt the task physically, but abort the process unless it was truly necessary and curtail the events that would unfold if you were actually willing to send the complete information along your nervous system to your hands. This may therefore involve an involuntary 'twitch' or movement of the associated digits are placed into preparation of function, processed against reality and then given the signal to not actually act.
In hypnosis, this may be circumvented by dissociating the particular thought-process-response-abort of a digit or entire limb; and therefore give control to the unconscious mind to enable (by suggestion) the route of conscious thought-unconscious process-conscious process (of the fact)-conscious response (do not act)- unconscious response (dissociate from conscious)- ignore abortive conscious attempt- unconscious unabort... Which would then display a reaction or response in a physical manner or behavioral context.
Body language may be considered the most commonly visible aspect of IMR, but may also include such unconscious activities as doodling or art - as the conscious thought is sublimated into a different type of activity in unconscious expression.