as Rodney has said never sharpen the flat side .keep it flat . it will cause you to dig into the wood where you don't want to and you wont have full control .Its important to maintain the correct angle which will vary depending on the person and is always a matter of debate .You usually find your own way of both cutting and sharpening to suit you
Its like scissors or shears they will not cut if you try to sharpen the flat side
Its like scissors or shears they will not cut if you try to sharpen the flat side