You can place one or several vectors. You can set their direction and their strength. They will act ont the corresponding area of the image. In this window you can place your vectors. By clicking on the Add button, you add a vector at center of the window, whereas clicking with the Middle Mouse Button puts it where you click. Vectors are red when selected, and gray when they are not, with a white point at tip. Angle: Direction of the selected vector. This slider has the same action as right-clicking (see above). Strength: This slider acts on the influence domain of the selected vector. This influence lowers with distance. Strength is showed with the vector length. Strength Exp.: This slider acts on the length of ALL vectors, and so changes the strength of all brush strokes. Voronoi: A Voronoi’s diagram consists in partitioning a plane with n master points into n polygons where each polygon has only one of these n master points and where any given other point of the polygon is closer to the master point than to any other. So each polygon limit is midway between two master point. Here is an example of a Voronoi’s diagram: Here, when this option is checked, only the vector closest to a given point of the image influences this point. You have there some types to arrange the brush strokes within the selected vector domain. Describing them is difficult, but you can see the result in the Preview. Source.