+1. That's the overrun clutch in operation. When decelerating, like coming off plane when you cut the engine throttle, the engine rpms can drop but the prop rotation is locked to the water passing across it. The clutch allows the prop to slow as the boat slows regardless of how much slower the engine rpms are. Without it, when you cut the throttle the prop would cut accordingly and you could wind up with your teeth in your windshield if you had one. Grin.
Mark