That detail can then be compared against a lower subdivision is still being generated by ZSpheres as opposed to being you are using onscreen.
Detail is added to an object at a very high subdivision level possibly with many millions of polygons. All subdivision levels are the same topology, at different levels of resolution. Now, when you go back to higher levels of subdivision, the throbbing veins and almost invisible muscle striations will still be visible! This will allow you to toggle that detail on and off of the original, but may limit what you can do with the mesh.
