(For a larger, more detailed photo of my drawing please visit my flickr site).
One of the differences between the GCA method of modeling form and other methods I've been taught has to do with where visual illusions take place. Previously I've been instructed to copy values as closely as possible. The problem with this, though, is that often the values that we see are actually illusory values. Take for instance the visual illusion of Mach bands. Mach bands are strips of light (or dark) that seem to occur on the border between areas of light (or dark) value and the gradient between them.
|(Notice the illusory strip of light on the left of the gradient and strip of dark on the right)|
I first encountered these as demonstrations of optical illusions in psych class, but recently I found what I think is an example in real life on my cast. See how the bottom edge of lower lip of the neck seems to be lighter than the upper edge? This may partly be due to reflected light -- you can definitely see reflected light from the floor more on the left side of the lip of the neck than the right side -- but look closely within that dark lip in shadow. The part that curves up to the upward-facing area of the chest seems get darker right before it gets light, while the lowest edge seems to have a thin light strip of light right before the dark shadow on the wall. It doesn't come out well in photographs, but I think much of that experience is probably due to something like the Mach band illusion.
So, if I were to copy values, the illusion would happen in my eye and I'd draw the illusory experience I had on my paper: I would draw the lowest edge of the cast with a thin strip of light, and a darker strip of shadow before the edge turned up into the upward-facing chest area. But I know the value on that shadow edge should be about the same (other than perhaps a bit of reflected light) -- so with the GCA method I didn't draw it, and instead drew an almost uniform dark value on that strip. However, the illusion still happens! (Or it should, if I've drawn it right). It just happens in your eye when you view my drawing, rather than in my eye before I draw it.
It's the difference between drawing my visual experience versus setting something up that will create a visual experience for the viewer. And something about letting the "mixing" happen in the viewer's eye -- setting the stage for an optical illusion to take place, rather than recording an optical illusion -- seems to make the drawing more realistic.