the upper grille block forces more air to go over the car, which has less resistance than the air going under the car. That is what i read somewhere here. The lower grille block will probably help too, but since you are now forcing air to go under the car then the gains are lower since it is more turbulent down there.
Sorry i couldn't find any references i tried searching but i remember reading something about it here
some expert can chime in if they know
cheers
Marko