Why does Paul start to ride his rocking horse in the first place?
The house and family are haunted by the phrase "there must be more money." They all hear this phrase and silently acknowledge it to each other. Neither the mother nor the father can seem to make enough money to satisfy this household urge for more money. One day, Paul asks his mother why they don't have a family car. She replies that they are poor. Paul asks why they are poor. She replies, "it's because your father has no luck." In a twisted bit of logic, Paul's mother tells him that luck is what causes someone to have money. Paul replies that he is a lucky person. Paul's mother congratulates him but it is clear that she does not believe him.
Paul goes off, determined to find luck, believing that this will bring him to money. He tries to think about where to find luck. In the process, he would ride his rocking-horse and get lost in his dreams and determination to find luck. This process becomes like a trance. Riding the horse, he is trying to will himself into some mental state in which he can find luck. Magically, this results in his ability to pick winners of horse races. And this, of course, results in money.