This is the heartbeat of the drivetrain. The aftermarket transfer cases are responsible for translating and multiplying the engine's power in order to make it available to the rest of the drivetrain. In an automatic transmission, the torque converter is the first step in transmitting this power. A manual transmission uses a mechanical clutch and flywheel. Next is the complex set of gears in the transmission. These are either shifted by the driver (manual) or built fluid pressure inside the transmission (automatic). Different gear ratios allow the vehicle to operate as efficiently and still provide power for acceleration. The transmission's power is transferred to the output shaft, which allows for the next stage of power delivery.