The story's name comes from the rocking-horse that the protagonist Paul rides and which gives him knowledge of which horse is going to win the next race.
Once he's obtained this knowledge he puts a bet on the horse and, without exception, wins a lot of money. Unfortunately, Paul's mother Hester is a very greedy woman, and no amount of money is ever enough for her. No matter how much Paul wins through betting on horses, she always wants more. And so, no sooner has he collected his winnings, than Paul is back on the rocking-horse, where he hopes that the name of the winning horse in the next race will come to him once more.
Hester's greed ultimately leads to tragedy as Paul falls off the rocking-horse and dies. But not before he comes up with Malabar, the name of the horse destined to win the next race. Paul only wanted to please his mother, but because she was never satisfied, no matter how much money Paul brought into the house, it was always inevitable that he would never succeed. Hester has always felt that she's lacked something in her life. And now, thanks to her insatiable greed, she lacks a son as well.