Michael Delaney on Aug/15/02 said:basic high school geometry and math :
it's the circumference of a circle and how the radius determines circumference.
if you reduce the radius on a pulley (UNDERpulley), the distance for 1 revolution (circumference) is lowered. So the for example on a PS pulley, there is a certain amount of power used to turn one revolution. If you make the distance to travel for 1 revolution smaller, the power needed to complete it will be less. This frees up power to be used to turn the crankshaft instead. Less parasitic power loss. Obviously an overpulley has the opposite effect.