Attempting to further close loopholes in the regulations, the FIA have removed the possibility for teams to gain a substantial aerodynamic benefit from the starter motor hole in the diffuser. A first regulation change in this area was implemented back in 2010 after teams started to create unusually shaped starters, allowing them to make a larger starter hole in the diffuser, and thereby extract more performance from it. Back then, the FIA stepped in, allowing the hole to be no larger than 3500mm². Any other part of the diffuser had to be a continuous shape, a result of the earlier ban on double diffusers.
It has now become clear that further measures were taken by scrapping the starter hole completely, requiring teams to either design a flap in the diffuser that would close itself, or otherwise leave an opening that is not visible from underneath the car or further than 350mm behind the rear wheel centre line. Clearly, most teams have gone for a flap, often metallic, as in Williams' case, enabling the starter engine to still reach the gearbox while complying with the rules in all other situations.
Mercedes on the other hand opted to create a U-shape in the centre of the diffuser. Obviously this still allows airflow through this gap and enhance the diffuser, but the effect is likely to be much less interesting than with the start holes of 2013 and before. In fact, the central starter hole was one of the main reasons why Red Bull's Adrian Newey designed tunnels underneath the RB9's exhaust ramps, as the ramps would otherwise block airflow towards the critical central part of the diffuser.
Note: even though there used to be a regulation proposal to enable F1 cars to start themselves by using the electrical energy stored in the ERS system, the rule was later dropped, requiring the use of a starter motor that brings the crankshaft up to speed before firing up the engine.