"Initially designed for open-spaces communications, the system is created of two devices wirelessly connected. Two persons can communicate to each other by knocking on the devices. Their knocks on a device are transformed in form changing movements and are transmitted to the other device. "
Reference:
Rasmussen, M. K. et al 2012 I'll Knock You When I'm Ready... Reflecting on Media Richness Beyond Bandwidth and Imitation. In DIS '12
http://cargocollective.com/ioanao/Knock-Knock