For all my iPhone-owning, guitar-playing friends:
Text from the YouTube page:
I just happened upon this trick when testing what it was like filming from inside my guitar. *Note this effect is due to the rolling shutter, which is non-representative of how strings actually vibrate.
• You must have the strings brightly backlit to get the camera to capture at such a high frame rate (pure conjecture). You can see how the effect fades when the buildings come into view.
• Use a pencil
*This was used with the front facing camera, try the back camera, it may capture better!
Here's another video, that shows the same "Rolling Shutter" effect:
This is a still image, which shows the effect:
cc licensed ( BY NC ) flickr photo shared by jasons_show
"Virtually all consumer grade digital cameras, including cell phones, do not take the picture instantly when you push the shutter button. Instead, they quickly scan over the CCD CMOS sensor from the top left to the bottom right, like the electron beam in an old CRT television. This is called rolling shutter capture. This scanning process is fast, but sometimes it's not fast enough." (TUAW)
Try taking pictures of very fast moving objects (airplane and helicopter propellers, fans, bicycle wheel spokes, etc.) and see what the camera captures.
You can read more about rolling shutter at Wikipedia, and at TUAW.