When the 2010 McLaren MP4-26 broke cover pre-season, rival teams' attention was immediately drawn to the letterbox intake on the top of the front of the chassis. This was the first glimpse of the f-duct, which became the big technical topic of discussion that season and played a part in the introduction of the DRS in 2011.
The concept of the f-duct was elegantly simple. The intake was used to channel air through the car and directed to the rear wing. But when a hole in the cockpit was covered - in the case of the McLaren by the driver's elbow - this airflow was used to blow across the upper element of the rear wing to stall it, slashing drag.
While there was some strong public opposition to the f-duct on the basis of the oft-referred to ban on moveable aerodynamic devices, it wasn't illegal.