[Sidewinder]

Couple things:

First, the purpose of the spoiler is to create downforce. Think of a dragster. The very reason why they have these wings on the front and rear is to create downforce. Of course, the body has a great deal to do with the downforce as well, as Harvey noted. For an example, look at the venturi effect.

As Chike noted, the spoiler appears to not be in the shape of an airfoil. Nevertheless, if there are pressure differences between the upper and lower portions of the spoiler itself, then we have some forces present.

With an airplane, lower air pressures are found above the wing while higher pressures are under the wing. The high pressure "lifts" the wing and the airplane flies. The same effect is used to roll the airplane. When the airplane rolls to the right, an aileron on the left wing drops down, changing the pressures while the aileron on the right wing rises, again changing pressures. The right wing develops less lift while the left wing develops more lift. This rotational effect allows the airplane to roll on it's lengthwise axis.

Anyway, back to the spoiler. All we need is higher pressure above the spoiler and lower pressure beneath it and we could have downforce. I am willing to bet that as the air stream that flows off the rear window and beneath the spoiler is travelling faster than the air flowing on the stream higher (the stream that flows over the spoiler). Therefore, despite the fact that the spoiler is not shaped like a normal airfoil, we could still get downforce.