Never heard of this. I would check the linkage to insure the tranny is fully engaged when in forward and reverse. Maybe you have worn clutches inside the transmission and when enough force is applied to the prop, it starts to spin. The engine cannot turn over from the prop moving through the water unless you have extremely low compression, in which case you would notice lack of power when motoring. You should not sail with the clutch linkage engaged in the forward position. This can damage the transmission. Always sail in neutral or lock the prop by shifting to reverse. Again, if the prop is spinning while sailing and the transmission is in reverse, this would indicate to me that the clutch must be worn out.