Definitely worth a read in it's entirety; here's the excerpt on the "moveable" blown front axle.
An aero trick raced since 2012 has been the blown front axle. This uses the brake ducts to purposely direct air out through the hollow front axle, though not for any cooling or braking effect. Instead, this is part of the front wing's efforts to push the turbulent front tyre wake away from the rest of the car. This outwash effect created by the blown axle and front wing endplate helps keep the rear wing and diffuser in clean air, allowing them to work more efficiently. If the tyre wake is allowed to hit the rear bodywork, then less downforce will be created.
Many teams have run these blown axles since Red Bull introduced them, and have simply become another trick in a team's aero-armoury. There has been nothing remarkable or interesting in their development in the past few years. However, another FIA technical directive released mid-season was aimed at Ferrari and its ducting system, allowing the axles only to be blown in turns.
That Ferrari could achieve this, in a potentially legal way, came as a big surprise to other teams and media alike. Additionally, there were questions as to how it was able to keep this secret, when the ducts and axles are so exposed during the car build-up process on a Wednesday and Thursday at Grands Prix. It transpires that an engineer left Ferrari for another team and the trick was revealed, with immediate queries from their new team to the FIA in a clarification request if this was legal. Clearly the FIA response was negative and Ferrari was told to remove the ducted set-up.
Of course, moveable bodywork is banned and any simple means to close the duct into the hollow axle when the car is steered would have been evident to the scrutineers and rivals, so the effect had to be more covert. A paddock source confirmed that Ferrari was using this trick and that movable flap valves were used inside the brake duct to create the on-off effect. In a straight line, the duct would be closed and no flow would be passed through the axle, but as the car starts to turn and the lateral G-force builds up, the flap valves opened under the load and the duct started blowing through the axle.
What Ferrari was trying to achieve was a mix of downforce and drag reduction. On the straights, the unblown axles would do less to prevent the tyre wake upsetting the rear aero, so the rear wing ran in dirty air, creating less downforce, but crucially less drag. Then through the corners, the flaps open and the duct blows through the axles, pushing the tyre wake away from the rear end and making it create downforce again.
Quite how Ferrari sought to justify this system will no doubt remain a secret, perhaps citing the set-up as part of the suspension, therefore responding to car loads and thus escaping the movable bodywork regulations.
Regardless, it was rightfully outlawed and has been removed from the car since mid-season, and this will have cost Ferrari some efficiency.