For a PB system of $k\xaf<0.40$ without strain mismatch, no crease manifests on the film surface, and the Ruga-phase evolves along the pathway (SM1 → DM1 → QM1) as shown in the bottom row of Fig. 6(b). For PBs with negligible viscosity, unloading follows the bottom row configurations (QM1 → DM1 → SM1) either with or without hysteresis, depending on the stiffness ratio. However, for an appreciably viscoelastic PB system of $k\xaf<0.40$ with strain mismatch, P-period switching makes the ground modes, DM1 and QM1 set off to SM2 and DM2, respectively, during unloading. Various possible P-period switching transitions are illustrated in Fig. 6(b). We denote these modes by SM*i*, DM*i*, and QM*i*, $i=1,2,3,\u2026,$ for which modal translational symmetry holds in every single, double, and quadruple primary period(s). The index, *i*, stands for the ratio of the primary period of the mode with respect to the primary period of the ground mode. For example, the set-off modes SM2, DM2, and QM2 have the same geometric mode configurations as the ground modes SM1, DM1, and QM1, correspondingly; however, the primary period is doubled. When SM2 generated by P-period switching of DM1in the unloading cycle is reloaded by compression, the mode will bifurcate to DM2 instead of returning back to DM1. The energy barrier is too high to directly flip the convex curvature of the SM1 wrinkle peak to a concave curvature of a wrinkle valley. Therefore, it is likely that if the viscosity is properly matched for the loading rate, the wrinkle modes would diverge under cyclic loading, following SM1 → DM1 → SM2 → DM2 → SM4 → DM4. This mechanism can excite the ground mode (SM1, DM1, and QM1) to a higher energy modes (SM*i*, DM*i*, and QM*i*, *i* = 2 or 4,…) by tuning the loading rate for a given viscosity.