Preparing a motion by first going into the opposite direction to build up momentum is called anticipation.
The anticipation position creates a larger distance for applying directional force (like the swing of an arm during a throw). When the force stops (e.g. the ball leaves the hand during a throw) the object has gained momentum driving it into the desired direction.
Anticipation also improves readability of an animation by indicating to the audience in which direction something fast is about to happen.
Anticipation is one of the 12 principles of animation.