Have you ever noticed how much the silhouette of the crossfire looks like an airplane wing?

Basically, (I'll try to keep this simple) there's more than enough downforce on the front of the car at speed. The deployment of the spoiler just equalizes the downforce on the back of the vehicle to counteract the lift generated by a low pressure zone created by airflow over the car.

If you're curious about it's effectiveness........disable the spoiler and go for a drive on the expressway at speed! You can feel the back of the car try to "hunt" around......it's a less than comforting sensation! Don't say I didn't warn you!