It's explained on his website, though perhaps not super clearly. The composition is a fugue, so it has a subject (main recurring melody). Each occurrence of the

It's explained on his website, though perhaps not super clearly. The composition is a fugue, so it has a subject (main recurring melody). Each occurrence of the subject is represented by a large box on the timeline, and when it plays, a slow column moves along the box. Also notice the many countersubjects (secondary recurring melodies). They are made clearly visible in the video: wherever they appear, their notes are highlighted and connected so that each occurrence looks like a small, recognizable constellation. It might have been interesting to treat all subject and countersubjects equally with the "constellation" style (which I find clearer than the weird box-and-column), then try representing harmonic aspects of the composition, which are so far completely left out of the visualization. Not that it would be an easy task, of course...