How do spoilers on sports cars help reduce lift?
They produce downforce in the same way that 'plane wings produce lift - just in reverse. The shape of the spoiler causes the airflow to be faster on the top than on the bottom, thus causing downforce.
The other person was mostly right, except he had the actual physics backwards. The airplane wing is shaped so that the air above it is moving faster than the air below, which means there is less pressure on top of the wing, eventually leading to lift to pick the plane off the ground. A spoiler on a car works the exact same way but the opposite. The bottom side of the wing plane for the car spoiler is longer than the top side, so the air moving under the bottom of the spoiler is moving faster and thus has lower pressure. This forces the rear of the car to not lift at higher speeds (since a car itself is shaped like an airplane wing with a flat bottom and rounded top, so the air travels farther over the top of the car than under it, creating lift at higher speeds).
