In this video, a guy sticks his iPhone inside his guitar while its recording and then starts to play. And then the strings go crazy.
What we're seeing is not actually how the strings look, obviously. It's caused by the rolling shutter effect, which is when a digital camera scans across the frame either horizontally or vertically rather than capturing the entire frame at once. So parts of the frame have to play catch up, and we're left with guitar strings making crazy waves and shapes. Awesome!