The aerodynamic function of a NASCAR race car or truck can dramatically impact competition on the sport's fastest tracks. It can also make teammates out of unlikely rivals seeking an advantage.
The process of "drafting" has been in practice since the superspeedway boom of the 1960s, when it was discovered that two cars moving in close nose-to-tail formation could cut through the air easier than one car by itself. The mutually beneficial partnership reduces the amount of turbulent ("dirty") air around multiple cars, allowing all cars in the draft to make small but important gains in speed.
Even 50 years later, the use of the aerodynamic slipstream is still an important facet of high-speed competition. The increased efficiency of air flow helped lead to the popularity of the tandem draft, which caused a vacuum effect behind the lead car, allowing a closely following car to move with much less resistance in the air, thus more speed. The extra speed of the second car makes drafting also beneficial to the leading car, as multiple cars can travel faster than a single car. The introduction of the Generation-6 car, however, helped minimize the effectiveness of the tandem draft, reviving pack racing.
See the graphic below to see the difference clean air makes during a NASCAR race: