  We give a unified description of the flip-flop effect in spinning binary
black holes and the anti-alignment instability in terms of real and imaginary
flip-flop frequencies. We find that this instability is only effective for mass
ratios $0.5<q<1$. We provide analytic expressions that determine the region of
parameter space for which the instability occurs in terms of maps of the mass
ratio and spin magnitudes $(q,\alpha_1,\alpha_2)$. This restricts the priors of
parameter estimation techniques for the observation of gravitational waves from
binary black holes and it is relevant for astrophysical modeling and final
recoil computations of such binary systems.
