The principle is simple...animate a sack of flour that conveys different kinds of emotion and realism. It's a simple idea, but a clever one because it forces you to think outside facial expressions and use body language combined with different animation techniques to put the emotion over to the viewer. I think this is going to be a valuable skill and one that I will be practising for the next little while.
This is a scan from the book...there are many drawings of the sack with a description underneath each of the emotion. They are so good though that you don't need to read the description, you know what the sack is feeling, which is the idea.