I really love doing this activity because it helps students to be less precious about their work and gives them a chance to have a laugh with each other at the end of the lesson.
I got the idea from reading Betty Edward's "Drawing on the Right Side of the Brain." She encourages students to practice this method of drawing as it forces them to focus on the lines and shapes that make up a picture rather than informing their drawings on preconceived ideas of what objects look like. I handed out print-outs of Picasso's drawing of a man sitting in a chair and a knight on a horse and asked them to turn them upside down and draw from the top of the page downwards. I think the results are great!