A spin is an equilibrium state in which the aerodynamic forces tending to oppose roll, pitch and yaw rates are balanced by gyroscopic forces tending to increase them. The gyroscopic forces arise because the aircraft is rotating about all 3 axes at once. Thus roll rate, combined with precession about the yaw axis, gives rise to a torque about the pitch axis. Yaw rate, combined with a pitch rate, gives rise to a torque about the roll axis and so on. Hence the motion is self sustaining. All 3 rates can become oscillatory, particularly when the roll inertia is low compared to the yaw and pitch inertia. In the Hunter, for example, roll reversals are common in the spin (to the tune of 3 sick bags in my case). In the Jaguar spin all 3 axes are oscillatory and while the spin looks like a totally random falling leaf the rates recorded by on board gyros are all nice regular oscillations - all at different frequencies.