The basic principal is at low speed the car has a lot less downforce, so the car is running a fairly high ride height because there is little downforce pushing it towards the ground. Because of the higher ride height these special wishbones are close to touching the floor of the car. Something about the shape of these wishbones creates low pressure (downforce.)
When the car gets up to speed, the downforce acting on the car pushes the chassis towards the ground (lowering ride height.) This creates a gap between the special wishbones and the floor of the car, ruining the low pressure, and decreasing drag. Thus increasing top speed.
Basically McLaren is getting loads of downforce from it at low speed, but very little drag at high speed.