In some old windmills blades were made up of wooden grill instead of a solid plank or sail. How did it work ?

You tube has quite selection of these old windmills. Here you can see one up close and notice that each of the blades is not simply a flat stick. So some of these were pieces to catch the wind. A complete sail would catch more of the wind. a href="http://youtube.com/watch?v=izbqIUu88yg&feature=related" rel="nofollow"http://youtube.com/watch?v=izbqIUu88yg&f…/a