tangent shading?

what is tangent shading and what does it do?

Images are better then words.
First one is with normal shading, second one is with tangentshading.


IMHO It can be used to get some anisotropic effect, like in brushed metals.

Checkout the wiki: http://wiki.blender.org/index.php/Manual/Raytraced_Reflections
it has more of a description and a good example image. Not that musks wasn’t also correct.
Good luck.