Those Gestures are Whack
The paper, Whack Gestures, describes research into creating simple, minimally disruptive gestures for cellphones. The researchers were able to create a series of gestures based on a combination of whacks to the phone. Most smartphones have built-in accelerometers that can detect the motion from a whack. The aim was to design gestures that would allow users to silence the phone or ignore/answer calls without removing the phone from a belt holster or pocket.