Gurney Flaps - What are they and where can they go?

[Tozza Mazza]

Hi all,
With the current limitation on aerodynamic bodywork, the gurney flap is more important now than it was pre 2009. I was wondering how they work, how big they can be and where can they go on the car. For example, how is the size of a gurney on the trailing edge of the diffuser limited?
Thanks, TM.