Rolling shutter is a method of image acquisition in which each frame is recorded not from a snapshot of a single point in time, but rather by scanning across the frame either vertically or horizontally. In other words, not all parts of the image are recorded at exactly the same time.
— Wikipedia: Rolling Shutter

If you have an iPhone, or any other device with a rolling shutter, try out this technique yourself. More inspiration can be found in our search.

Photos from jasons_show, Right Brain | Chris Piazza, tossthecam, and Ben Bunch.