I've just been trying to understand this. I believe the trick is in how with Adam7 there are progressively more pixels displayed with each pass. This allows you to make big changes from one pass to the next, even from almost completely white to almost completely black.

If you then interrupt the connection at the right point - directly after one pass - you can get these animations. But unlike a gif, once loaded they are no longer animated since they are done loading.

Edit: Also important here is the cache policy ('Cache-Control: no-store').