By fabricating a cold air box around the carburators or fuel injection system the air being forced into the upper scoops could only go into the intake. The engine can only pull so much air, leaving the area above the box with a positive pressure. Once the air box is pressurized, then no more air can travel into the scoops so it will pass around the car instead of into the rear clip area. This may be the best way to keep the rear clip from blowing open at speed, and the ram air effect improves performance. Every original GT40 I have seen has some sort of air box around the intake, so maybe Ford was on to something.