how does Buck begin to undermine Spitz's authority?

Buck and Spitz go at it for most of the chapter. Buck is a physical presence and Spitz knows this. The whole alpha-dog thing clicks in and there is friction. There are quite a few altercations where Buck undermines Spitz. At one point Buck undermines Spitz’s leadership by siding with the weaker dogs when Spitz tries to discipline them. By the end of the chapter Buck puts some fancy moves on Spitz (the old rush and fake move) and defeats his rival.