The back of the Williams is most like that of the Ferrari F138 chassis, but perhaps even more compact. The aero fin extends well back from the engine cover but the engine cover ends well ahead of the rear differential, making for a truly wasp-ish profile. That major focus of work on the car has been the incorporation and refinement of the Coanda-channel exhaust. Williams said it was hesitant to introduce it last year because of the loss in power that results, but the solution they've come up with has limited the power reduction while maintaining aero benefits. The only problem now is that the FIA has told the team, and Caterham, that their solutions might not be legal – they can run the exhaust in testing, but come Australia it might be declared out of bounds.
You can see the FW35 in action and listen to the team discuss its development in the video below.