Sketch of the mechanism of the VI done in parallel-plane geometry illustrating a small part of the shell. The structure of the SNR is modeled by a hot bubble (left part) and a distorted thin dense shell (middle) that expands in the ISM (right) at the horizontal velocity V _s. In this perturbed configuration, the thermal force, F_th, and the ram force, F_ram, are not colinear, so that matter flows (white arrows) from the extended front zone (A), called “hill”, to the receded shell zone (B), called “valley”.