Notice how far the rear wheel is compressed up into the fender?

In the late 90s, teams would drop the rear end of the car as far as possible.

By doing so, the rear spoiler would be "hidden" from the air flowing over the windshield and roof.

This was done during qualifying at Daytona and Talladega to maximize top-end speed at the sacrifice of handling.

During the race, they would return the car to a normal position as the ability to draft off of other cars reduced the need to minimize the rear spoiler.

Additionally, to pass other cars, handling becomes more important than top-end speed, requiring more down force.

"Dirty Air" is a difficult problem to solve in all motorsports.

Dirty air is air that following cars experience do to lead cars disturbing the air in front of them. 

In a way, the lead car is "punching" through the air creating a low pressure wake behind it.

It's considered "dirty" because the disturbed, low pressure air makes it difficult for the following car to maintain full downforce. 

Without full downforce, the following car can't maintain full speed through corners, making it difficult to pass.