Here's the back of my RC funny car showing an "aid" to evacuate air out the back of the car:
And same from my True 10.5/Pro Stock
Mind you, the funny car goes 90+mph and weighs at 48oz (3lbs) and hasn't been airborne at those speeds. That gives you an idea how effective low pressure under the body is. Also note that the angle of the wickerbill is not that extreme: just enough to be effective and not create a lot of resistance.