I have a Yanmar with a Kamzaki/Yanmar transmission. My 3-blade Flex-O-Fold closes when I put the gear in reverse, and then back in neutral. I marked the prop shaft so that I can turn it by hand to a position with two of the blades folded in the up position, and one blade down. (I guess that is only practical if you have easy access to the prop shaft. Mine is conveniently located at the bottom of the companion way steps.) My theory is that that will prevent the prop from opening at very low speed. I guess it still could happen if you miss a tack and get caught in stays. But then it would happen no matter whether the shaft is locked or not. Anyway, I have not caught a lobster float in the three summers that I spent in Maine since I installed this prop. On my first trip to Maine I had a fixed three blade prop, and caught several floats with toggles.