If you can imagine looking at a slice through the middle of the car. The flow over\under the front wing wants to rise up and form a rising wake before flattening off. However the nose and chassis above the wing obstruct this wake forming and reduce the front wings efficiency. High noses and raised chassis improve the wake formation, but always the nose will impact on the flow. As high pressure builds up above the flap, this can’t be released and hence flattens the low pressure flow under the wing and also spills around the sides of the car needing the various flow flatteners that the front of all the cars are adorned with. What Ferrari have done is provide a route for the high pressure to bleed into, I suspect the hole starts and ends before the trailing edge of the flap. The air exits over the top of the car and probably splits left\right to pass over the sidepods. Meanwhile underneath, the low pressure under the wing is not affected by the high pressure region and is allowed to expand to create more downforce. The effect should both create more downforce and more rear downforce. However the effect will be small, I don’t see this as a must have development worth whole tenths for any car, merely a solution for Ferraris overall aero philosophy.
One side note, this solution might deemed legal under the 2009 non-overlapping bodywork rule, making it a useful development when other aero aids are banned.  | 
Linear Mode
