No such thing as a free lunch, and a prop is an airfoil just like a wing...
A windmilling prop is a essentially...a windmill. It's extracting energy from the slipstream and turning it into compression heat in the pistons and internal moving parts friction in the motor. The slipstream energy being consumed by the turning (but dead) motor has to come from somewhere, and that somewhere is the kinetic energy of the airplane. You can trade altitude (potential energy) for kinetic energy to maintain airspeed, and that would be second-nature for a pilot. So net result is higher descent rate.
A non-rotating prop creates some drag, but it's mainly form drag since the prop blade must have significant flow over the airfoil to transfer energy to or from the slipstream. The blade is stalled, and stalled airfoils do not transfer much energy or generate much lift. A prop blade rotating at speed with airflow along the chord is designed to be very efficient at energy transfer but only if the air is flowing the right direction.
More in depth than that will require some math...