The Gurney flap, pioneered by American automobile racing icon Dan Gurney (who was challenged to do so by fellow American racer Bobby Unser), was a right-angle piece of sheet metal, rigidly fixed to the top trailing edge of the rear wing on his open-wheel racing cars of the early 1970s.

Daniel Sexton Gurney (April 13, 1931 – January 14, 2018) was an American racing driver, race car constructor, and team owner who reached racing's highest levels starting in 1958.

The device was installed pointing upward to increase downforce generated by the wing, improving traction. He field-tested it and found that it allowed a car to negotiate turns at higher speed, while also achieving higher speed in the straight sections of the track.

To conceal his true intent, Gurney deceived inquisitive competitors by telling them the blunted trailing edge was intended to prevent injury and damage when pushing the car by hand. Some copied the design and some of them even attempted to improve upon it by pointing the flap downward, which actually hurt performance.

Gurney was able to use the device in racing for several years before its true purpose became known.

More Info: en.wikipedia.org