You know what synecdoche is? It's basically when a part represents a whole—and in "Rules of the Game," Waverly's hair totally represents the girl that it's attached to.
Waverly's hair is first described as "disobedient" (9) and "thick" (9), both words we think her mom would happily use to describe her daughter. And importantly, every morning Waverly's mom fights her hair's natural inclinations, pulling and tugging until "she had formed two tightly wound pigtails" (9). You know what else Waverly's mom refuses to let run wild and do its own thing? That's right, her children, especially Waverly.
Waverly's hair comes up again when she has her picture in Life magazine, wearing "neatly plaited braids clipped with plastic barrettes trimmed with rhinestones" (49). At this point, Waverly is doing everything her mom desires—coming home from school, practicing chess nonstop, and kicking butt and taking names when she plays the game. The tidiness of her hair stands in for the fact that Waverly has fallen in line, at least as far as Mom's concerned.