From the creator's site:
Driven by hand crank through a small planetary transmission, the rotating camshaft and roller lifters cause the balls to "roll uphill". The balls then deposit in the "dropper", which directs them to the holding tilt-track (a feature borrowed from the Rolling Ball clock). When the sixth ball arrives, the track unbalances and the balls roll down the coaster track to start their journey again.
Here's a link to more info on this cool wooden ball stairway machine.