In the event of an engine failure both the engine and the propeller will slow down. The CSU will attempt to maintain the selected RPM by driving the propeller blades to a fine position that will continue until reaching the fine pitch stop.
Despite the engine failing the airplane still has forward speed, for a single-engine airplane due to the glide descent the forward speed causes the propeller to have a negative angle of attack due to the direction of the relative airflow. This angle of attack generates sufficient torque to rotate the unpowered propeller in the normal direction of rotation.
The component of the force at right angles to the plane of rotation now acts rearwards and is no longer thrust but is now drag and presents a flat disc to the direction of travel.
Because the propeller is being driven only by the relative airflow it is said to be wind milling. As such, it causes a considerable amount of drag, reduces the IAS, reduces the Lift/Drag ratio and increases the rate of descent.