In this example I've tried to only outline the very strongest changes of plane, and use broken or implied line for the softer changes of plane. Deciding where to put the implied line is fairly easy with the side of the nose and the shape of the mouth. It's more tricky with the very gradual changes across a rounded cheek or chin. Sometimes in these areas, couple of short marks will just suggest the contour every so slightly.
So as you can see, implied line, in conjunction with an awareness of change of plane, can help you create a more natural and three-dimensional look in your line drawings.
