Snapshots of the terms M₃₁τ′ and M₃₂u′ that appear in the equation for the azimuthal velocity perturbation and which must be sufficiently in phase for the viscous overstability mechanism to work. The snapshots are from integrations with λ = 200 m, β = 1.35, and cover one orbital period in equal time-intervals. With increasing strength of the satellite perturbation (quantified through the nonlinearity parameter q) these terms become increasingly out of phase. For q = 0.4 almost all possible phase differences in the range 0–2π occur, which explains the negative growth rates of overstable modes on all wavelengths (Fig. 11, lower right panel).For clarity the two quantities have been rescaled so as to possess equal amplitudes in all plots.