Shadow spraying is essentially spray painting without any masking. The term comes from the effect that is created when the paint is sprayed at an angle in such a way as to not cover the entire area that is being painted. Think of the sun setting in the afternoon and the shadow you make on the ground. If the sun was the paint source and the light around you was the paint, it would cover everything except where your shadow fell. It's a neat trick, and if you have some old bodies lying around give it a try!