This is a model of an outboard engine gear case. Below, i'll explain how it works for someone that doesn't know.
The power from the crankshaft of the engine is driven to the gear on the top "pinion" through the driveshaft, which is fited with a nut below the pinion. The pinion then delivers the power (rotational) to the 2 gears next to it. The gears are rotating with opposite rotation. (the pinion and gears are rotating all the time the engine is powerd on...) Then with a lever (which usually is placed on the left side of the outboard or to your remote control) the clutch which is between the gears, is moved to the desired direction. Then the clutch is fited with the gear (because of its shape) and as the clutch is mounted on the propeller shaft, it rotates the propeller, which eventually moves the boat. If we want to put in the other gear (reverse or forward) we move the clutch to the opposite gear. When the clutch is in the middle position, your engine is in neutral. All gears as long as the pinion and the clutch as placed inside the lower Unit of the engine and are sank into oil (to reduce friction and wear).