This is "bump mapping done wrong", performed for visual effect but without consideration to correct vector math, and restricted to a plane viewed along its normal. It's not the general way to do bump mapping in object space, or even the correct way to do it in texture space with any kind of physically relevant lighting model, because we compute neither the reflection vector nor the "halfway" vector. This is merely a quick hack to show the utility of the exact analytic gradient for this kind of effect, but the visual impression is undoubtedly that of the surface of bubbly-blobby almost-luminescent green goo that moves in an eerie manner as if it were alive. (Or my imagination may be running amok. It's late.)
For a proper example of bump mapping (and displacement mapping) with correct recalculation of normals, on an arbitrary curved surface viewed at any angle, we refer to the tutorial on the 3-D version of "psrdnoise". Forget you saw this. Really. Or, well, remember the image, but please forget the code. Proper bump mapping is not done like this. We are skipping quite a few steps here.
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