Originally Posted by Av8Roadster
On the coupe the air flows over the roof, like a wing, when the spoiler is deployed it [like the flaps on a wing] extends the area of the roof which increases the drag, however when you extend the flaps on a wing it creates a greater low pressure area above the wing and increases lift. That's why you deploy them, you get greater control [lift] at slower speeds.
Flaps extend the top area of the wing to provide increased lift. A spoiler is kind of like an "anti-flap". It is used to decrease or "spoil" the lift that a wing generates. It does that by popping-up and interrupting the high-speed airflow over the top of the wing. You can see this when an aircraft is landing. As soon as the wheels touch the runway the pilot will pop the spoilers up and activate the thrust reversers on the jet engines to kill lift and reduce speed as quickly as possible.
Our spoilers are angled just right to kill some of the lift that the body produces and also collect enough high pressure air on their top side to add some useable downforce without adding too much drag.
If you're interested,
here is a link from NASA's Beginners Guide to Aerodynamics that shows what I'm talking about.