Actually, now that I've tried to finish up my previous artwork, it doesn't do what I expected it to.
When smudging a highlight, the tool effectively erases the highlight. The darker color smudges on top of the lighter color, and the lighter color doesn't smudge anywhere. That, and there's a weird "burn" effect on the edges of a smudge when smudging certain colors (areas of shadow, reddish colors, etc.)
It was a good try, though. Might still be helpful, I'llhave to play with it a bit more.