Flappers. The whole purpose of a shroud is to give the fans the ability to draw air from the entire core. Air is like water and electricity, it follows the path of least resistance. Open holes in your shroud will allow high pressure air from the grille opening to flow through the radiator when travelling at higher speeds without the restriction of the shroud creating an air pocket to slow it down. In low flow situations when you rely on the vacuum from the fans to pull the air through the radiator core it will draw air from the path of least resistance. If you have open holes in your shroud it will be easier to draw air from there than to pull through the radiator core. If your shroud sits 1/2" off of the radiator core it will be easier to draw air through that gap than through the radiator core (don't laugh... we have all seen it.) The flaps serve to close these vents when there is vacuum inside the shroud forcing the air to be drawn through the core thus increasing the efficiency of the cooling system.
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