Stellar velocity dispersion plot as a function of bulge mass for SDSS central galaxies. This plot is restricted to disk-dominated galaxies which present face-on (b/a > 0.9), in addition to all bulge-dominated galaxies. The plot is subdivided into small hexagons which display the quenched fraction within each region of the parameter space, as labelled by the colour bar. The median relation (±1σ) is shown as a solid (dashed) black line. The correlation strength between σ_⋆ − M_B (ρ_S) and the rms dispersion (σ) are both displayed on the plot. The region in which velocity dispersion is poorly constrained by the SDSS data is shaded in grey. Clearly, the optimal route to maximise quenching through this 2D plane is through increasing σ_⋆ (not M_B). This provides a simple visual confirmation of the primary result from the RF analysis in the right panel of Fig. 4.