Here we have our unserviceable lawnmower with its engine exposed.
That finned device on top is the flywheel. But not only. This is multifunctional. Those fins do what you might guess and act as a cooling fan for the aircooled engine immediately below. But they do more than that. They are part of the push mower equivalent of cruise control. As the engine spins the fins blow air onto that plastic vane you can see almost touching the left hand side of the flywheel. The faster the engine turns the more air pressure is applied to the vane and it gets pushed away from the flywheel. It pivots and so the other end is pulled towards the engine. It’s attached to the throttle and tends to close the throttle as revs increase so it acts as a governor, keeping speed constant. Start cutting heavy grass and the load on the engine increases. It slows down, less air is produced and the vane moves back in, thus opening the throttle and compensating for the extra load.
So there you have it. The lawnmower equivalent of cruise control.