This is from Kim Wilson's technique cards.
MASKING
Stamp your foreground first and stamp it again on some scrap paper or a sticky note. Trim the image on the scrap exactly, or slightly smaller. Layer this mask on top of the foreground image. Stamp your background on top. The background will appear behind the foreground image.
My addition to this-You can also do this to add more of an item to a page and it looks like they are in front of one another. I used it for a group of roses or strawberries in some of my projects. It really looked neat.
Gallery at Splitcoaststampers
If you click on the resources button at the top of this page, you will come across many, many step-by-step directions from the masters on this site. It's great!