The wings of a bird or an aircraft are the most obvious producers of lift. But lift doesn't necessarily mean an upward force countering gravity. In fact, downforce is a form of lift -- negative lift.
Lift is the aerodynamic force perpendicular to the direction of the body in motion. Conversely, drag is a resisting force parallel to, but coming opposite from, the moving object. Lift -- colloquially called a skyward force -- is usually present to one degree or another in a moving object. Because lift and downforce are opposing forces, part of the effort to build a stock car with a strong downforce involves overcoming lift.
The engineering goals are to curtail the amount of air flowing underneath the chassis to ensure a closer attraction between the tires and the ground and to provide easy escape for air that does get underneath.
Stock cars are characteristically designed with rake -- meaning the car's rear is higher off the ground than the front end of the chassis. It keeps the pressure underneath the car down, preventing lift.
Spoilers, front air dams and wings produce this effect. An air dam is mounted underneath the front bumper to block air flow underneath the body. Wing appendages, used on Formula One and Indy cars, are turned upside-down to provide downforce instead of lift.
Race cars occasionally become airborne despite these devices. The danger is especially present when a car is spinning, which radically alters the aerodynamic forces in play. During a high-speed spin, air can move rapidly enough over the roof and hood to produce a powerful lift force.
Several safety innovations are installed on NASCAR vehicles for such emergencies, such as a recessed right-side window. Stock cars circling oval tracks to the left are more likely to show their right face in a spin. The sharp edge to the right-side window deflects air instead of letting it flow freely over the roof. Flaps recessed into the car's roof, another safety feature, begin to rise if air pressure suddenly drops above the car, blocking air flow [Source: Leslie-Pelecky].