Williams have been trying out different rear bodywork on their FW33 car. The new layout is quite similar to what can be seen on the Red Bull RB7 as it creates a hole above and ahead of the beam wing to allow hot air get out of the sidepods.
With the new exit, the team was able to close the exit low above the diffuser, aiming to create a cleaner airflow onto the diffuser, instead of using the hot air that went through the radiators upstream in the sidepods.
What makes this so interesting and perhaps difficult to get right is Williams' notorious low gearbox. With other teams developing high and narrow gearboxes that rely on air passing along the sides of the gearbox, Williams attempted to get more air above the gearbox. While this new layout is just testing, CFD analysis within the team's headquarters must have shown that clean air flowing around the sidepods is more useful for downforce generation at the back compared to hot and somewhat more turbulent air from within the sidepods. That is also the reason why Red Bull are doing a similar thing since the very introduction of their RB7.