What does this equation mean?

The magnitude is the first surprise. In a driven or flowing lattice, the cone speed along the drift is $c_{\mathrm{eff}}$ + $v_d$ in one direction and $c_{\mathrm{eff}}$ - $v_d$ in the other, so the anisotropy between the along-drift and transverse directions is $\delta$ c/c $\simeq$ $v_d$/$c_{\mathrm{eff}}$ — first order in the drift velocity, not the crushing second-order $v^2$/$c^2$ $\sim$ 10^{-8} that made the 1887 experiment so demanding to run and so easy to see nothing in. At a tenth of the cone speed, the anisotropy is ten percent — a signal any competent interferometer would catch on its first pass, the mirror image of the historical null result. Such anisotropies are not a fiction of the model: ultrasonic measurements already resolve a field-angle-dependent sound velocity in the Weyl semimetal tantalum arsenide, a real, measured anisotropic cone speed in exactly the material family the paper’s own census uses for its Weyl-semimetal row [ 13 ] .