On the front cover in particular, devices can be used to reveal a hidden message, or to reveal the full picture. This can be done in a number of ways, through slide-outs, additional print outs to be placed over front covers or belly bands to name a few.
This is a design for the book design awards. Using a printed device on the front cover, the message 'don't judge' extends to 'don't judge a book by its cover.' A clever play on words, and something that has context within the event that it's representing.