Mitchell Starc is sometimes chucking

I have noticed that when Mitchell Starc is given the task of cleaning up the lower order, he is chucking or throwing the ball to bowl fast. The Laws of Cricket specify that a bowler’s arm must not extend during the bowling action. Only the rotation of the shoulder can be used to impart velocity to the ball. But from his bowling action sometimes he is not using the rotation of shoulder to impart velocity to the ball but instead using his ball holding hand to impart velocity.