Ground effects
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Ground effects are parts of a vehicle's lower body designed to enclose the gap between the bottom of the fuselage and the ground. The purpose of ground effects are to deflect air around the car rather than under it. Air passing under a vehicle exerts a lifting effect at high speed, which reduces the tires' contact with the road, resulting in loss of control. Ground effects can be part of the vehicle's original design, or aftermarket parts added later (body kit).
Reducing the distance between the road surface and the vehicle under-body causes the air flow between the gap to increase in velocity. Air pressure reduces inversely with flow speed so this effect creates a vacuum underneath the vehicle, essentially "sucking" the car onto the ground and increasing traction. This can be dangerous if the air flow underneath the car is suddenly lost, by "bottoming out" or hitting a bump, the sudden drop in traction can easily cause the driver to lose control; this is documented as potentially one of the contributing causes of the crash which killed Formula One world champion Ayrton Senna.
