Several of my students have pointed me to this video: http://bit.ly/1N1IGR8 Apparently the guy put his cell phone inside his guitar and took a video. My initial reaction was that it looked fake, because the time scale of the movement of his thumb seemed mismatched to the time scale of the motion of the wave impulses on the string, but now I'm thinking that this may just be a stroboscopic effect. Would anyone care to put together a more detailed interpretation of what's going on? I wonder what the frame rate of cell phone video is. Some of the comments on the video may be relevant.