The blade stop limits flapping of folding propellers if used in pusher setups, where the forward flapping blades could otherwise collide with fuselage or wing trailing edge. It works with (and was specifically designed for) this spinner and motor: https://hobbyking.com/en_us/folding-propeller-w-hub-35mm-3mm-shaft-8x4-5-2pcs.html and http://www.dys.hk/ProductShow.asp?ID=30.