Both the audio and visuals are generated using the same math, rendered at different speeds so the eye can see. No tricks or audio files are used: everything is calculated from raw sine waves, with a theory of how energy is released in a pluck. I've been working on this side project for a while now. Since 2012 I've been tinkering with digital audio synthesis with a focus on the trigonometry of timbre. I've previously made http://tonehack.com where you can create timbres using scratch, and the exploration of the topic has led to building accurate realtime string simulations on the GPU, which became Pluck.