What do the highlighted blocks and moving "pistons" within mean (see [1])?

[1] https://www.youtube.com/watch?v=Tybs1T9OLBg&list=PLtj_HurkS7...

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...

That's the fugue ( https://en.wikipedia.org/wiki/Fugue ).

> In music, a fugue (/fjuːɡ/ fewg) is a contrapuntal compositional technique in two or more voices, built on a subject (a musical theme) that is introduced at the beginning in imitation (repetition at different pitches) and which recurs frequently in the course of the composition.

The box (fugue) reoccurs throughout the piece and reoccurs several times in various variations.

Looking at the bottom (zoomed out map area), you can see the fugue as the "bolder" sections of the piece and their shape is clear that it repeats throughout the piece. There are also times (single pistons) where the sequence occurs outside of the fugue proper.