Spa-Francorchamps is the first medium downforce track on the 2009 calendar, so teams are coming up with smaller rear and front wings to reduce drag and increase top speeds. Williams for one is special, as it didn't only make its rear wing smaller, it is now also lower.
The arrow in the image clearly shows that the main flap is not extending to the maximum height - which is the upside of the end plate - by a few centimeters below. The solution puts the wing slightly more behind the car's body and hence creates a little bit less drag.
The idea itself isn't new and has already been used by Honda last year.