The advent of Formula 1's Drag Reduction System in 2011 forced designers to rethink their philosophies behind rear wing design. But things haven't changed as much as most people think.
Under current FIA rules, a rear wing has to fit into a rectangular box when viewed from the side, and has a maximum of two closed aerodynamic sections, the main (lower) wing plane, and the upper flap; each with a specified width and depth.
In order to get maximum performance from the wing, it is essential that the slot between the plane and the flap is as far back as is possible to still enable stable attachment of the airflow to the wing. Too far back and the air will separate before the slot, meaning no downforce at all. Too far forward and the underside surface air speed will not increase enough, meaning that potential wing performance is given away.