Are you familiar with the Spotlighting Technique? Often in doing this technique, you stamp a larger image in black, then stamp a portion of it again in black. Leaving the main image in black only, you color in the smaller portion. After the coloring is completed, the creator uses a punch or a die to cut out only the colored portion, usually in a circle or square. A narrow border in the same shape is often added.
Awhile back I'd stamped a group of Christmas-y characters, coloring in only part the Santa, which appeared in the approximate center of the image. By adding the colored Santa over the black and white Santa, it gives that portion of the image more focus and importance. You can see this card here
While I didn't do exactly that with this card, the effect is the same.