Our first impulse when we wish to do a thing is to make the muscular movement necessary to the accomplishment of the action proceeding from the thought.
But we may be restrained from making the movement by another thought, which shows us the desireability of repressing the action.
We may be inflamed with anger and may experience a desire to strike the person causing the anger.
The thought is scarecely formed in our mind before the first steps toward striking are taken.
But before the muscle moves our better judgement causes us to send a repressing impulse (all in a fraction of a second) and the opposite set of muscles holds back the action of the first set.
The double action, ordering and countermanding, happens so quickly that the mind cannot grasp any sense of motion – but nevertheless the muscle had begun to quiver with the striking impulse by the time the restraining impulse held back the movement.