Basically, you just put a 'stamp' on a piece of digital paper, which shows what city you are in - then you 'fold' the paper into an airplane and make a little 'throwing' motion with your phone, being careful not to let go and fling your phone across the room, and the plane flies off. Then everyone can make a sweeping motion with your phone to catch other people's planes (seemingly random) to view the stamps, and add your own, then throw it back.

Chrome -> Developer Tools -> Device Toolbar is your friend to send some paper planes :)

You stamp a paper with your location, "fold it" and then throw it using the accelerometer of the phone.

Then you can catch other planes, see who else has stamped it, add your own stamp and throw it back with the others. Quite neat.