Surely that would just create more drag?
After reading up on it, they're supposed to reduce drag. If there's a steep change in surfaces, e.g. where the rear screen suddenly dips down, then instead of the air cleanly following the surface of the car, it breaks away and becomes turbulent. The "gap" that this creates in the air can try and "suck" the car back, creating drag.
The idea of vortex generators is that they disrupt the air flow slightly and the vortexes actually make the air follow that steep change of direction better, reducing drag.
However, if you look at the links I posted, the reduction in drag is absolutely tiny even on cars where the vortex generators were specifically designed for them.