An image distortion effect caused by the way camera shutters operate at very high shutter speeds. Beyond a certain speed, focal plane shutters, as used in most interchangeable lens cameras, change the way they work. Instead of exposing the whole sensor at once, they expose it in a narrow strip between two shutter curtains passing very quickly across the sensor. This means that if a subject is moving very rapidly it may take on a skewed or twisted shape. This can be apparent not just in stills photography but in video too.
This also happens with electronic shutters, so that although they offer even higher shutter speeds than mechanical shutters, they too capture image data in ‘strips’, and while they may offer an exposure time of, say, 1/32,000sec, it actually takes considerably longer for this electronic scanning process to complete. This can only be fixed by so-called global shutters, which capture data from the entire sensor area at once.