here are three different drawing of "They heard a knock at the door..."
The emphasis was on the pattern of lights a darks within the picture to create tension, movement, interest. To help convey that, it appears that he then put tracing paper over the drawings and shaded in the darks, knowing that he could then remove the top layer with just the fascinating negative/positive patterns to make his point.
His handwritten instructions on what to learn from this exercise are insightful and interesting. And timeless. And if you can't get enough, here is another blog post on Fawcett by Apatoff for you to enjoy.