An internal combustion engine can only produce power within a certain speed range. Typically, engines have a torque band of approximately 2500 to 4500 rpm.
To make use of the narrow torque band, a collection of reduction gears are used to keep the engine within its peak operating range as the vehicle speed increases and decreases. The gears inside the transaxle are configured in a "planetary" manner. Varying drive ratios are achieved through the use of hydraulically actuated clutch packs and bands that manipulate the planetary gears.