First of all it creates a homogeneous base. Spread a few drops of foundation all over your face and add a thin layer of powder to even out.
A suitable contouring must be studied for each face shape.
For example, to "shorten" an oval face, darken the cheekbones with earth and then illuminate the forehead, the area under the eyes and the center of the chin.
For an edgy face, put the dark powder on the top of the forehead, near the hairline, under the cheekbones and on the jaws.
For a round face, first darken the temples, the outline of the chin and the contours of the oval and then brighten the center of the chin and the area under the eyes.