You know how the top fashion models have great cheekbones that make their faces look both refined and a little fierce at the same time? They're relying on more than nature to get that chiseled look. They're likely using a tinted powder for shading, and a contour brush to distribute the color without leaving a telltale stripe behind. Somewhat like a blusher, a contouring brush helps add definition to the face, but this is more about suggesting an underlying structure than adding a healthy glow. If you have a fuller face that could use some planes and hollows, skillfully applied contouring will help give you the illusion of high, prominent cheekbones and a firm, well-defined jawline.
The brush -- With firmer bristles than a blusher brush, a contour brush usually also has an angled tip. There is some overlap between blusher and contour brushes, so experiment with a few to find a style that works for you. The angle on the head of the brush helps define the more precise lines used in contouring with a bronzer product. The firmer bristles then help blend the powder so the line disappears, but the slight shading remains.
The technique -- Always tap excess powder off the brush before applying it below your cheekbones or jaw. Position the brush with the bottom bristles facing the direction in which your hand is moving, so you're working with the flow of the brush and not against it. A contour brush is also effective at creating a more chiseled nose by adding shading on either side of the nose just below the bridge. You can use a contour brush to apply blusher, too, but if you think your features need better definition, using a specialized brush for each task is a good idea.