Shutter adopts special push-pull,open- close device, ensure shutter totally open and close, wind-proof, rain-proof and dust-proof. Centritugal type open machanism can fuarantee that shutters completely open and close.Make the shutters opened to reduce resistance.increase the air flow. - See more at: http://www.lufeng-china.com/product-show.php?contentid=24#sthash.j3XHHCC8.dpuf