Now, I could tell you that what makes this composition such a tour-de-force is its simple complexity. A crab canon, or cancrizan, is sort of like a round. A melody starts off, and then the second voice comes in a few beats later, and this single tune layered against itself creates the harmony (like "Row, Row, Row Your Boat).
But a crab canon has the second voice enter with the melody in reverse. So the art is to construct a tune that makes sense forwards and backwards - and can harmonize with itself in the process. Hard to understand? I agree. That's why I like this video. View, and enjoy.