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An underdrive pulley is on the crank, which is the source of power for all of your other accessories. By slowing having a smaller pulley on the crank, each of the accessories turns less per revolution of the crank. This is what frees up the power. An overdrive pulley is something that is used on one of your accessories, most likely the alternator. this is done to keep the voltage output up, especially if you have a big sound system in your car. Usually none of this is a problem though. And it has nothing to do with how you drive your car at all...other than ~10hp/tq.
