Vertical side view of 3D-integrated streamlines (left) going through the circled-star-position in Fig. 3. We note that here we show the y–z plane. Streamlines are coloured according to starting height for purposes of distinguishing them. The circled star (bottom of the plot) is at the same 3D coordinates as before. A flow separation occurs at around z = 0.41r_H dividing the streamlines into strongly shocked upwards and strongly shocked downwards flows. Scanning through horizontally adjacent streamline families reveals that this phenomenon is responsible for leaving the entire column volume behind the shock devoid of streamlines with the ability to replenish the missing mass. Evolution of the free-fall region at the vertical black line (right). Density profiles are shown evolving in time at the position indicated by the line in the left figure. The formation timescale of the FFR can be read-off as ≈2 Ω⁻¹. See Fig. 6 for a density map during the formation process.