by ducting, I think he means blocking off a portion of the front of your car. it SEEMS to be illogical, but from what I understand, By having such a large opening, at higher speeds, the pressures inside the engine compartment increase, causing the air in front of the radiator to stall and slow down, reducing the cooling efficiency. By reducing the size of the opening, you reduce the amount of air entering the area, BUT at the same time, you reduce the pressure in the engine bay, so instead of pushing air through the radiator, the air gets pulled through the radiator due to the reduced pressures, and increasing the cooling efficiency. Ever notice how small the front opening is on newer cars? In order for it to work the front of the car needs to be totally air tight except for around the radiator.