This is an amazingly awesome example of an effect known as rolling shutter. You can see it on most cameras, but it is most pronounced in video. Why does it happen? When the shutter on the camera you are using activates, it moves from one side of your chip to another, copying the scene bit by bit instead of all at once. When your subject is moving, in this case the strings in the guitar, the sensor in your camera puts together an almost liquid image, thereby giving the ‘rolling shutter’ effect. In still photography you can see this effect when you take a photo using a flash combined with a shutter that is faster than recommended (125ish). As you can see in the image below, half of the image is exposed to the flash and the other is not. This is due to the flash and the camera shutter not being in sync and operating at the same time. Here is a quick example I took in my office:
Its hard to explain how this happens, so here are two more videos to show how SLR shutters work!
The first video is a bit techie for most people, but the info is 100% necessary for any one who calls themselves a photographer to know!
Skip to 1:04 to see how the shutter curtain works, below you will see the action in slow motion. (Awesome!!)