I've seen bamboo rods made with spaces between the cork rings in the handle, with the idea of allowing the rod to flex the whole way down, without the handle interfering. Whether that made any difference, I don't know.
But, from the video it looks like this is different than just trying to get the rod to flex the whole way down.
It looks like the rod is attached just at the very butt end, and that the attachment allows the rod to pivot, i.e. swing freely back and forth within the hollow handle, for a short distance, before it hits the inside of the hollow handle.
The idea, I think, is that the rod will move faster this way, because it is unconstrained by your fingers. And if the rod moves faster, it will create greater line speed.
It seems somewhat similar to the idea of the "power snap" where at the very end of the forward casting stroke, you push down with the thumb and squeeze with the fingers, creating a forward rotation that creates speed at the rod tip that is additional to what you get from the forward arm motion.
