Pick what you want to see, and the surface you want to see it on. The machine computes the field you have to stare at first. Hold the dot, don't blink more than you must, and let the colour arrive.
Drag on the stage to paint. Coarse cells on purpose — an afterimage is low-resolution and gone in seconds, so bold shapes are the only ones that survive the trip.
Strength is the adaptation gain. It is a knob and not a constant because it depends on your screen, the room and how long you actually held still.