Okay so you can’t really see guitar strings vibrate like this in real life…. basically the way an iphone records video leads to you being able to ‘visualize’ the actual vibrations.
The iPhone captures movies with a strange, floppy jello-like appearance. This is called rolling shutter and, like most digital glitches, can be exploited to great effect.
When you shoot video with the iPhone, its CMOS sensor captures images by scanning one line of the frame at a time. If anything is moving fast, then it will be in a different place as each line is captured. This can lead to weird distortions in still photos, and to rather odd effects in video