This is a complex but often practiced technique of increasing the yield from a rough diamond by a diamond cutter. There is a lot of discussion in the trade about painting and digging out, and it can affect the overall light performance significantly if either is significant.
Basically, the technique involves adjusting certain angles of the final faceting of the crown or pavilion, either inward or outward. This makes the facet junctions a little less pronounced, and thus gains a little extra weight from the rough. These angles are important to the flow of light internally however, so it affects Symmetry and overall Light Performance.
Here is a great article by GIA about the subject ::
Here is an image that describes the technique in the simplest terms ::