There are straps you place in the door opening then close the door on them to secure them. The videos I've found show it fairly stable in wind (probably not hurricane) and bricks being dropped on it (interesting to me because we have pines cones that fall from 60' pine trees... she has dents...
Sorry I know this is an older thread but relevant for me since I started using a car cover on the wife's car and it is a lot of work.. There is no way to cover an SUV without dragging the cover across the paint when your by yourself. Luckily the top is all glass but that still leaves the hood...