Red Bull are saying now that it helps with the stability of the car on entry to corners - under braking, in other words. In my view, that is all it can do. It acts as a dirty great rudder sticking into the airstream, except that it's not designed to turn either way. I can't really see it having much effect on the rear wing and it's unlikely that it helps to clear up the wake of the airbox.
Kamm's Law states that airflow will break away from a rearward facing slope of the car once the angle between the horizontal and the slope exceeds 7 degrees. Turn that sideways and you have the amount by which the airboxes may come together at the rear. Otherwise the air would break away into eddies and you'd have the dreaded drag. I can't see cars that have been in the windtunnel ignoring such a simple way of avoiding drag.
Unless nobody told Honda about Professor Kamm...