(didn't check the source) It looks like x,y and time as a seed for a function that paints (additively) a complicated line. click 1 spot over and over, quickly

(didn't check the source) It looks like x,y and time as a seed for a function that paints (additively) a complicated line. click 1 spot over and over, quickly - you'll get a white line that gets a little thicker as it's fully saturated. click a different spot over and over, but hold it a little longer, you'll see the delta-t effect tracing the same pattern over and over. one way to implement such a thing, you'd want a function to look at a pixel and it's nearby neighbors, and brighten. red = red+10 or some such. next you'd want a linedraw function, given x,y,t - pick a spot near x and y, step a few pixels away in some direction, brighten the stuff between. repeat that, with a bias tward the direction you're travelling, and cap it out some how. something like steps=(x+y+t)%100 +10. min length is 10, max is 100, kinda vary with time. Anyway, that's the basics. you can futz around with the specific functions to give a nice effect. edit ah, mynegation's comment is probably the truth. particles with inertia would do just the right thing for biasing direction.