When you purchase the book, it comes with an ENT membership card with 170 numbers on the back. Then you go to any restaurant listed in the book for a "buy one entree, get one free" special and they'll punch the cooresponding number on the back (as to prevent multiple use of the card at one location). Many restaurants, however, do not punch the card, thereby allowing you to use it over again at a restaurant.

Very handy system.

aloha