Stock, there are little motor-driven doors in the plates between the intake manifold and the head which stay partially closed at low rpm to increase the velocity of the intake air in order to maximize fuel vaporization, providing (supposedly) smoother running, more power and improved economy at lower rpm. At higher rpm, they are wide open, but still cause some level of restriction. The charge motion plates you can buy are wide open (having no movable doors) and should allow better airflow and therefore add some hp.