Piggyback labels have two layers and a liner. The top layer is a facestock with adhesive; the second layer has a release coating on the front (to allow the top label to be pulled off) and adhesive on the back. The two layers are pulled off the liner and affixed to a form (or card, or letter, etc.). The top layer can later be pulled off by the end-user to be applied to another surface.
These labels make it a cinch for recipient responses and interaction. They can be premiums, too – reminders, quick references (important phone numbers, websites, etc.), or games.