If the ellipsoid is flattened in one direction only, like a pancake (that is, if \lambda_3 is much smaller than both \lambda_1 and \lambda_2), it means that the gradient directions are spread out but perpendicular to e_3; so that the isosurfaces tend to be like tubes parallel to that vector.
Finally, if the ellipsoid is roughly spherical (that is, if \lambda_1\approx\lambda_2\approx\lambda_3), it means that the gradient directions in the window are more or less evenly distributed, with no marked preference; so that the function I is mostly isotropic in that neighborhood.