HI! I grant this post is OLD!!
I was learning from it - and attempted to add in a water layer and to make it look nice, i wanted to add a thin green layer just about the water surface to look like algae or something. But, when i added shaders, limited the height (+/- 2m from the water surface) I found that the green layer actually tracked the layer warps. Rather than the altitude limits being absolute, they seemed to follow the warps.
I wondered, in this set up, how i might apply a narrow band shader that was effectively flat.
Churz!