The idea behind it is to connect your sense of touch with your sight. The author says that the artist is to place his/her pencil on the page and then look at the object they are about to draw. He must try to convince himself that his pencil tip is touching a contour on the object (effectively a line following the surface of the object) instead of the paper. He must then move his eyes along the contour, along with his pencil, with the purpose of coordinating both movements. The only time he may look at the page is when he reaches the end of a contour and must reposition his pencil at the beginning of a new one. However, no lines may be drawn whilst looking. From what I can tell, the excercise is designed to prevent the artist from attempting to draw a shoe for example (we often draw what we think a shoe should look like, but not necessarily what is in front of us), but instead 'feel' his way around for the details that make up the shoe.
I found this really tricky. It requires a surprisingly large amount of concentration to do. Other forms of drawing tend to get to be second nature after a while, so you don't really think about what you are doing. However with contour drawings it is entirely the opposite: you must really be aware of every movement your pencil makes. Anyway, here are two of my attempts:
In case you can't see what they are, they are of an old boot drawn from two different angles. They look like a child's drawings, but bear in mind that I wasn't looking when I drew them. I made several other drawings, but there's no real point in posting them as they are not drawings to be viewed in the same sense as normal drawings. According to the book, the experience of drawing by 'touch' is the desired outcome, rather than a nice looking picture.