The animation is confusing...is one trip to the end and back the 1B iterations? I thought maybe they were counting up and once the slowest reached the end they would all stop but that didn't seem to be it. Maybe if it did (and there was a counter on each showing how many bounces they made), it'd be a little more clear.

Interesting nonetheless.

Looks like they are just bouncing back and forth at relative speeds to each other; the speed of each ball is all that matters. I also found this diagram confusing at first.