These are improved streamlined sails. The system used here is called Dekker sails. They need lesser wind, and since this windmill is not so far from the Belgian coast, they don't need to put canvas on the sails because they can profit from the inland sea breeze. Before starting the mill, they have to unlock the air brake system. When going to fast they will get active and slow down the sails. Previous image shows the miller unlocking the brake on another sail.