Stamp an image on cardstock. Stamp the same image on another piece of cardstock using the same ink color. Color in the second image, where ever you like. Punch or cut out a portion of the colored image. Mount the colored-in piece on a piece of coordinating cardstock (really helps make it pop). Mount the image over the uncolored image on the first piece of cardstock lining it up carefully. Clear as mud? This thread from
TLC06 might explain it better and give you lots of examples.
It's really pretty easy and a big WOW! I've done it with pencils, chalk, polished stone, and reverse spotlighting.