I think his mind started from the common example of a weight sitting on a table. There is a force of gravity one direction and an opposing force of the table pr

I think his mind started from the common example of a weight sitting on a table. There is a force of gravity one direction and an opposing force of the table pressing up against the weight keeping it where it is. For a coaster that isn't inverted you can say the same, gravity pulls one way then the track is pushing the other way. He just forgot it is reversed once the coaster is upside down.