What are the flaps that the drivers use in Nascar for?

May be...

They allow the air to pass through the car instead of lifting it up.

Blow on a piece of paper, then blow on a window screen. Same principle.

It's the small flaps on top of the cars( I think its two) that I have seen drivers open when they spin out or crash.

Those roof flaps (as they are called) deploy in order to create drag to keep the car from getting air borne.

they come up as a safety procaution. it helps slow the car down and it also keeps the car on 4 wheels and keeps it from flipping most of the time. although sometimes they do not work 100%

http://en.wikipedia.org/wiki/Roof_flap

They are called Roof Flaps, and they open by themselves when the car gets sideways or turned around.

The driver has no control over when they open or not.

Once the car is back going in the proper direction they close by them selves, usually.

it is to keep the car on the ground and not to flip over.

It is to allow the car to get air into it at high speeds to avoid them from getting airborne and flipping..car ferry seattle victoria