Someone had one at the Spruce Meadows AKC Summer Classic dog show, and they let me take some measurements and photo's on my iPhone, and I was able to reproduce this nifty leash.
Let's start with the handle. When the leash is at its 6 foot length, this is what the handle looks like. It is a fixed (stitched) handle with an mobile o-ring. Not too fancy yet, but just wait, it gets better!
Here is the trigger snap end with the leash at it's 6 foot length. Again there is another o-ring, but this one is fixed into place.
Okay, here's where it gets interesting... To convert from a 6 foot to a 3 foot, you feed the webbing through the o-ring on the trigger snap end.