The alt. and P/S pulley work by making the diameter of the pulley larger. Lightning the pullies helps too but the ticket is the larger diameter of the pulley.

It makes the moment to turn the assesory less, so in effect your car works less at running the Alt. and p/s. This energy instead of going into the devices in theory is used by the engine to better keep energy in the system. Thus keep more HP at the wheels.