If you want to extend your stock muffler(or any muffler) to behind your bike without too much expense or welding, that looks like one way to do it! The post doesn't contain any details as to how exactly it was constructed or how leaks are sealed, though.
UPDATE: The original photo was lost so I replaced it with another one that uses flex tube to have an extended muffler. Hopefully this will at least help to illustrate the idea behind the original hack. You can learn more about the muffler extension seen in the photo in this thread.