Found an example I made recently to play with vertex normals. Might help you visualize what I'm talking about. http://boards.polycount.net/showpost.php?p=1122194&postcount=34
Yeah it basically lets you modify the directions of the vertex normals. They control how each corner of each triangle will be lit, you can do some cool things with it. Works great for evening the lighting on planar foliage. Works great for killing the lighting seam between two adjacent meshes, like for example if terrain…