Particles steered by the picture, leaving brushstrokes that follow whatever is in it.
Each particle reads the brightness underneath it, turns that number into a compass heading, steps forward, and draws its trail. The picture is never drawn directly. It is used purely as a map of which way to go.
Reading brightness as a direction is what makes the strokes describe the subject. An area of similar tone produces similar headings, so every particle crossing it travels roughly parallel to the others, and the trails end up tracing out the shape of that area. Where the tone changes, the strokes turn. The result reads as deliberate brushwork rather than as scribble, and none of it was directed.
Each particle carries a small personal offset so that two in the same tone do not lie exactly on top of one another.