The goal is to attract the viewer's attention and then draw the eye around the piece along a smooth and comfortable path which ends back at the beginning. Though the examples below you black, gray and white, a similar effect can be done with contrasting colors or contrasting grain types.
The eye is drawn to dark areas first.
Example 1 pulls your eye to the center first and holds it until you eventually shift your focus outward, dark to light, light back to dark, to begin again.
Example 2 draws your attention progessively inward and then back outward to repeat the visual path again.
Example 3 focuses your attention in the center where it jumps back and forth between the black and the white areas, the gray border almost isn't noticed.
Example 4, like example 3 keeps your eye jumping back and forth between the black and white areas.
The goal is to attract the viewer's attention and then draw the eye around the piece along a smooth and comfortable path which ends back at the beginning.