These types of spoilers work by interrupting the flow of air along the top of the back window that would then flow downwards in a vortex towards the back/underside of the car which would create an uplift under the back wheels. By deflecting the air upwards, the downward vortex will not interact with the back of the car, thus it will not cause uplift, thus the backend of the car will be firmly planted at highway speeds.
It's not a 'wing' that generates low pressure on the lower side as its main purpose. (e.g. the inverse of what airplane wings do) That is why it's probably more correct to call it a spoiler...it spoils the uplifting affects of the high speed air coming down the back window that would create low pressure under the car.
If I'm not mistaken....!
