what exactly does this do?

 

the function of an underdrive pully is to slow down the accesories by ~25%. This means that you reduce the amount of power required to drive them. By doing this you recover some of the energy used to drive the accesories. to be honest unless you are running a very modified engine you will not see much of a difference by doing this modification. Also cars that are runnign large amps will have issues with power consumption vs power generated by the alternator....

Fro