Seems like a slightly more complex (has a binary -> decimal decoder) of this [1] which was made around the same time but is a bit easier to see how it operates (since it isn't made of K'nex)
[1] https://www.youtube.com/watch?v=GcDshWmhF4A
I guess the big difference is the ball-drop adders don't require electricity or motors for one calculation, but you need some method (human or otherwise) to move the balls back to the top. Also you can only carry out calculation downwards, so your CPU might wind up being a few hundred meters tall!
Not to disparage the ball-drop adders at all, just pointing out the differences.
