Black and white areas and simple energetic lines are basic visual tools which can be used to suggest very complex forms.
Form is expressed only by noting where the light is and where it isn't. Everything in between is midtone. Usually, if there is a simple light setup, midtones can be subtly hinted at or skipped altogether.
In my example image, we are drawn to a central figure. The figure's head is popping out from behind his shield, in contrast to the other figures in the image. Our eye will see the other figures and quickly note their similarity, treating them almost as a single mass. The eye will stop occasionally on one or two heads but will return back to the central figure to dwell in the action.