Time evolution of the radial velocity of the SN shock, (solid red), and of the maximum radial velocity, v_max(Ni) (dashed blue), on the surface where the mass fraction of ⁵⁶Ni plus the n-rich tracer X equals 3%, for models W15-2-cw, N20-4-cw, L15-1-cw, and B15-1-pw, respectively. The vertical dotted and dashed-dotted lines mark the times when the shock crosses the C+O/He and He/H interfaces, respectively. These times are identical to those of the snapshots shown in the first and second columns of Fig. 7.