Designers of marble fountains who don't use computing to design the paths run into reliability issues: sometimes balls derailing out of their track. They have to observe the contraption, identify problems (balls getting jammed up or jumping out) and then guess at the root causes and make manual adjustments.
That's the thing here: he has it running for hours presumably without any ball jumping out.
Most of the tracks consist of two rails, so the ball has two contact points. I'm no physicist but it seems like the goal would be to have ideally nearly equal forces at the two contact points at all times during the ball's descent. In other words, the track has to be perfectly banked so that the gravity and centripetal acceleration vector are balanced by a normal vector perpendicular to the rails. During a derailment, the ball has to lift away from one of the two contact points, so the normal force must have dropped to zero.
That's the thing here: he has it running for hours presumably without any ball jumping out.
Most of the tracks consist of two rails, so the ball has two contact points. I'm no physicist but it seems like the goal would be to have ideally nearly equal forces at the two contact points at all times during the ball's descent. In other words, the track has to be perfectly banked so that the gravity and centripetal acceleration vector are balanced by a normal vector perpendicular to the rails. During a derailment, the ball has to lift away from one of the two contact points, so the normal force must have dropped to zero.