This app lets you send images to your friends, but for your friends to see the image, first they have to send you another image! (And for you to see that image you also have to send an image back too!
So you see, this app puts you in a never ending image sharing paradigm with your friend. No matter what you do there is always one unseen image which needs to be unlocked by locking another new image.
Never ending fun! Full of surprises and perhaps frustration! :) Enjoy.