This is achieved by passing air through a slot in the rear wing (the black line near the bottom of the wing in the picture left), which neutralises the rear wing, cutting the drag. Such a device would also reduce the overall downforce, which would be a bad thing. So switching it on and off when needed on the straights is the key. That is where the question of legality comes in.
what you know about this?. So, analysis of more easily understand this
http://www.jamesallenonf1.com/wp-conten ... re-416.png