Net spine polarization angle as a function of a. Increasing a generically makes the location of the polarization swing move farther in, thus providing an useful diagnostic of spin. As with Figure 6, each curve has two markers in blue: face-up triangles denoting the prediction (Equation (34)) for the effective swing location along the background emission, and face-down triangles denoting the prediction for the effective swing location along the foreground emission. The location where the net EVPA = 45° lies roughly between these two estimates of the swing location.