Evolution of the FFR as seen in velocity as streamline colour and density as background colour, with density contours to guide the eye. Snapshots are taken at intervals of 1.0 Ω⁻¹. Symbols along the spiral arm shock surface denote the pressure ratio ζ, as defined in Eq. (3). Filled circles denote approximate ram-pressure equilibrium, i.e. ζ ≈ 1, and filled triangles indicate ζ > 1, i.e. the region where the streamlines can push past the shock. The ζ > 1 streamlines remain supersonic after encountering the shock. Those post-shock supersonic streamlines evacuate the envelope efficiently, which helps accelerate the flow further (note the intensification of the red colour on the streamlines between themiddle and right panels, which also corresponds to an increase of velocity in absolute numbers). We note also how the original spiral arm splits in two through this process.