I have been experimenting for the past day now with this. I like to think of it as selective lines or selective bolding of the lines. I start out by drawing the basic outline of the drawing and then adding small details like the eyes, mouth, lines in the face. I then go back and darken and bold only certain lines, to enhance and give the image the illusion of shading. On top of that I also make various shapes and fill them up with little lines. This also to enhance the drawing-- so that combined with the dark bold lines and the little sections of tiny lines-- when you stand back to look at this from a distance, you get somewhat of a sense of form or shape or dimension. This is the best I can do in this technique. It's not a perfect product, but at least it doesn't look totally flat. I leave the rest of the drawing with thin lines. Those thin lines are my foundation.
In memory of my dog. 1998-2003