The windproof catflap is shaped like a figure of 8 on its side. It is pivoted in the middle. It can rotate just under 180 degrees. Flanges on the edges of the flap prevent it rotating right round in the hole.
Whichever side the cat is on, the cat pushes on the right flap to get through.
There is a slight size difference between the 2 halves, which means when the wind blows it pushes the bigger right hand side a little harder, thus pushing it more tightly shut.
This was featured in a Bob Symes section on innovative products in the BBC's Tommorrow's World program. If anyone ever finds such a thing for sale, let us know where! Until then it belongs in the projects category.