Underdrive pulleys do just that, they underdrive engine driven accesories by changing the diameter of the pulleys. Smaller pulleys on the crank and/or larger pulleys on the accesories(power steering pump, alternator, etc). This makes the accesories spin slower, thus consuming less power. Beware however, underdriving your accessories too much can cause problems like not having enough power to keep your battery charged or run a large stereo system. If you drive relatively slow these are not for you. Only aggressive drivers should really consider these.