Not sure where I saw the idea for this, but the nub on the flywheel catches on the nub on the prop-shaft, causing the prop to spin. It allows for the powerplant to be removed without having to remove some form of linkage as well as letting the engine go through most of a rotation without load while changing direction
Old school coupling before Hucos became de riguor. Used to have two pins 180deg. apart on the disk and a through crossbar, whichever, put a rubber sleeve on one of the pins to keep it quiet when running. (Also saves a metal to metal signal generator).