It’s said to represent a racehorse,” Sylvia said. “Making that final spurt, the last effort in a race. The rider, too—the boy—you can see that he’s urging the horse on to the limit of its strength.
It also might be representing Carla and Sylvia. Carla making that "final spurt" to get away from Clark, and Sylvia urging her on to and past her limit?