This is a complex but often practiced technique of increasing the yield from a rough by a cutter. There is a lot of discussion in the trade about this, and it can affect the overall light performance significantly if the painting is significant.
Basically, the technique involved adjusting certain angles of the final faceting of the crown or Pavilion, either inward or outward, making the facet junctions a little less pronounced, and gaining 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 from GIA that describes the technique in the simplest terms ::