So that we know when to keep our fingers and toes out of the 'attack' zone

Well at least that is the more practical aspect of the butt shaking.... my cats are shaking their butts daily since I bought them little furry toy mice... To me, I have always viewed it as 'winding up their little hind feet'... like a toy clock mouse which one has to wind up before it sprints off.....

But whatever it is, butt shaking or feet winding, they look very cute and serious as they prepare themselves to pounce on their object of desire......