Fay Butler demonstrated this at a class I attended a few years ago. He found a radius gauge that fit the curve exactly. Then he moved the gauge up and down along the metal until the center of the gauge began to lift from the surface. At this point he made a mark at the end of the gauge. Then he moved over a couple of inches and repeated the procedure. He did this several times then he connected the dots (at the end of the sweep). This series of dots was what he called a "high light line". In the right light you can actually see this line along a body.
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