Most modern gearboxes are used to increase torque while reducing the speed of a prime mover output shaft (e.g. a motor drive shaft). This means that the output shaft of a gearbox will rotate at slower rate than the input shaft. This reduction in speed will produce a mechanical advantage, causing an increase in torque. A gearbox can be setup to do the opposite and provide an increase in shaft speed with a reduction of torque. Some of the simplest gearboxes merely change the physical direction in which power is transmitted.