YouTuber justkylevids throws his iPhone 4 inside his guitar and starts recording via the front-facing camera. As he explains:

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.

Tips:
• 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!

There is a heated discussion with *real science* over at reddit

The final outcome is pretty awesome, and as explained is due to the rolling shutter of the iPhone 4’s camera:

Mind. Blown.

[Mashable]