Chris Shier designed the interactive drawing HTML element one uses to make these.
You move your mouse or stylus around until you get a design you like and then screenshot the result.
Essentially it's a drip painting process that yields fluid, Naum Gabo-like structures melding the organic and the geometric. These also resemble photograms and William Wood paintings.
The geometry part is more apparent if you use the full-screen version on Shier's site - you can see the "wireframe" patterns comprising the swirls. I made these using a smaller window Shier posted on dump.fm and found I had more control with it.
What's good about this "virtual instrument" is also what's bad: it's almost impossible to make a poor design. Stubborn artist types want to do more than play, they want to break the boundaries of things and feel like they're doing at least some of the work.
In electronic music there's a better-understood distinction between "performer" and "instrument maker" (thanks to Paul B. Davis for pointed this out to me) than in so-called "generative" visual art -- or at least an acceptance of the blurring and overlapping of these roles. Hybrid drawings such as the above "challenge notions of authorship" or "raise interesting questions about artistic autonomy," depending on how tendentiously you want to phrase it.