It's funny that smaller balls don't display the tail wagging at all, yet it's stress that makes the breeding ones wag?
The tail waggin little ones do is similar to a rattle snake shaking it's tail, it is not a sweeping motion with organs out.
Wingedwolfpsion said it exactly as I have read it many places, and have has it described to me by numerous breeders, I just won't quote anyone without their permission.
The tail wagging, tail swishing the OP mentioned during breeding has zero to do with stress(IMO)