Underdrive pulleys do exactly that...they underdrive the crank pulley. It is like using the smallest sprocket between the pedals of a ten speed bike. Easier to turn, therefore freeing up horsepower.
The trade off... all other things being equal... is that now you are driving the belt slower per each revolution of the pulley, so you need to use slightly bigger pulleys on the waterpump and alternator, or you will underdrive them at idle and risk not charging and overheating Unless you live in a climate that constantly stresses the alternator or cooling system, I don't think there is a down side
