facing ratio creates the changing value, between 0 an 1. this value is creaed by the angle betwee the normal of the surface and the direction of the camera. i use it on anything that has transparency.
to test this u can create a plane and put a checker texture on it
make the texture bright colors, like red and white
then create a blinn, make the color 60%gray and make it 75% transparent. then render
now attatch a sampler info node's facing ration to the blinn's transparency RGB to see the difference