A "double diffuser" refers to a component in the rear of a race car that helps create more downforce or grip on the road. It is designed to improve the car's stability, cornering ability, and overall performance. Full definition
More carbon fiber is used in the rear double diffuser, a single piece of the lightweight material that starts under the rear axle. (slashgear.com)
In addition, active aerodynamics control a section of the underbody beneath the engine bay that automatically lowers 40 mm (at 50mph in Race mode and 75mph in Comfort, Sport and Sport +), to create a venturi effect that reduces front - axle lift by some 40 kg at 155mph as well as feeding the large double diffuser at the rear. (evo.co.uk)
The remarkable rise of Brawn GP from the ashes of Honda was the big story at the start of 2009 (along with double diffusers) but whilst Jenson Button completed a flawless lights - to - flag victory in Melbourne to kick off one of the most amazing stories in motor racing, behind him things weren't as straightforward. (wtf1.com)