Winfred,

Would a photocopy of the theory be helpful? Overbalancing is a very complex subject, which I may not be capable of explaining in terms that would be understandable.

You can easily prove that the prop position makes a difference on most gas engines. Mount the prop at 0, 45 and 90 degrees relative to top dead center. Crank the engine with the prop in each position and observe the tail vibration. Some engines will be worse than others. The magnitude of the vibration in X and Y will depend upon the crankshaft balancing strategy of the manufacturer and the ratio of crankshaft weight to prop weight.