It's a rather brilliant way to encourage readers to leave comments for the authors and artists of the exchange! As Madeleone said, you promise to donate a prize or two. In exchange you get bingo cards. You play by reading and reviewing the exchange gifts on your cards. You have to leave a comment before you can X out that box on your card. When you bingo you get to pick your prize from the donated prize lists. There were lots of fabulous prizes last year! It's fun!