Complex sentence: The "pole shift hypothesis" describes a change in location of these poles with respect to the underlying surface--a phenomenon distinct from the changes in axial orientation with respect to the plane of the ecliptic that are caused by precession and nutation.
Split into simpler sentences: The "pole shift hypothesis" refers to a change in location of these poles with respect to the underlying surface. Note that this is a different phenomenon than the changes in axial orientation with respect to the plane of the ecliptic that are caused by precession and nutation.