'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! (tried it, it didn't look as good for me)
There is a heated discussion with *real science* over at reddit.