Camera sensors, including those used in phones, use what’s called a ‘rolling shutter’ to capture video. Basically individual horizontal images are captured one after the other and then stitched together to create video that to the human eye is fluid. Problems with this image capturing system can occur when very fast movement is filmed, the classic example is aircraft props looking rather funky. Watch this video for a demonstration of the principle.
Anyway, onto this musical example. The man in the video places his iPhone 4 into the body of his acoustic guitar and then plays a few tunes. The resulting video is pretty interesting, the differences in vibration frequency between the strings can clearly be seen.