Underdrive
Think of it this way, underdrive means to slow down. Underdrive crank pulleys are designed to slow down engine accessories and reduce parasitic loss.
Crank pulleys, attached to the outside of the crankshaft, are the source of power for a car's accessories. The alternator and water pump are belt driven units, attached to the crank pulley, that use horsepower from an engine to provide their own services. This draw of power by these accessories is called parasitic loss. Underdrive crank pulleys increase an engine's throttle response and horsepower by reducing the power required to drive these external accessories. By replacing factory pulleys with smaller diameter units, the accessories are slowed enough so that their performance does not suffer, but just enough so that more horsepower is sent to the wheels and put to the ground and also benefit from increased accessory life and also deliver better fuel economy.