Days before the first Grand Prix of the season, McLaren's MP4/25 grew an air inlet on top of the chassis. It turned out to be a feeder for its F-duct system. An air channel is contructed through the chassis, along the driver's legs and through the shark fin into the rear wing. By carefully controlling the airflow with his knee, the driver can now cause the rear wing to stall, allowing up to 10km/h advantage on a straight.
When Jenson or Lewis put their knee onto the channel, they force the airflow to continue through the car and sharkfin and to exit on the rear side of the rear wing. This additional airflow disrupts the laminar flow along the underside of the rear wing, causing it to detach and effectively stalling the rear wing. This process momentarily reduces rear downforce and drag until the driver releases his knee and allows the air into the cockpit again.
As the system is obviously efficient, Sauber has already introduced their own version, while Red Bull Racing and the Scuderia have declared to be looking into a solution. Renault said 7 days ago it wasn't going to develop a similar system, but they might just have to if they want to keep up with the quick boys.