Schematic illustration of the four evolutionary stages of shock interaction with an extended CSM shell. Stage A: ionization of the CSM by radiation from the SBO at the progenitor’s surface. Stage B: radiative energy accumulation, in which a forward shock propagates through the optically thick CSM and radiation is trapped behind the shock front. Stage C: SBO of the extended CSM while photons diffuse outward through the unshocked CSM. Stage D: cooling phase after the shock has swept up the entire CSM shell.