Yeah it's not so easy to explain in a couple words. This comes from the Güllmar paper:

> "Variations of the conductivity, however, have a strong influence on the forward computation of the magnetic field as well as of the electric potential, as shown by Haueisen et al. (1997). Thus, comparison of electromagnetic simulations using an anisotropic model comprising conductivity tensors derived by using the direct transformation approach against the isotropic model would not only demonstrate the effect of anisotropy but would also include (unwanted) effects due to conductivity changes in the model. To compensate for the isotropic conductivity variations, the conductivity tensors were normalized. Assuming that the scalar conductivity represents the radius of the sphere of the corresponding isotropic tensor, the eigenvalues of the conductivity tensors calculated from the diffusion tensor data (see Eq. 3) were adjusted in such a way that the volume of the ellipsoid corresponded to the volume of the isotropic conductivity sphere ... 

*(equations...)*

> The ratios between the eigenvalues and thus the anisotropy ratio and the orientation (eigenvectors) of the tensors remain unchanged. In this work we will refer to this approach as direct transformation approach with volume normalization. This approach is similar to the proportional anisotropic ratio method proposed by Hallez et al. (2009), except for the formalism, which relates the diffusion tensor eigenvalues to the conductivity tensor eigenvalues..."