The knobs on the backplate are to rest the back end of the prop to them so the spinner lines up with the screw holes. The prop does spin counter clockwise when the engine is running and the knobs do not prevent the spinner from flying off. The only sure way to keep your prop from flying off is to tighten the prop securely to the engine. I use the prop wrench to get the prop tight then I follow up with a crescent wrench to make sure the prop is very tight against the backplate. Be careful not to tighten wooden props like this because it may crack the prop in the middle. Out of 40 plus flights I have not had one fly off yet (knock on wood). Just be sure when starting your engine to not stand in front of it (for many reasons) and set it to idle. Then get behind the plane and put your legs between the LE of the stab and the TE of the wing and rev her up.