Fluid
Drag across the water and the ink follows your finger. Let go and the swirls carry on turning by themselves, because the water is still moving long after you have stopped pushing it.
Swirl is the one to reach for first. With it at nothing the curls flatten out within a second and the ink drifts like smoke in treacle; wound up, small eddies feed off the big ones and the picture keeps churning long after you let go. Pressure passes decide whether the ink swirls or merely smears: too few and the water is allowed to pile up in places, which no liquid does.
Walls, detail and what the pointer does
Nothing flows through a wall and nothing sticks to it either, so you can draw a shape and watch the stream break around it and shed a trail of eddies behind. Walls stay put when you change what is running, and the stream mode lays down three of its own to start you off.
The ink is carried on a finer grid than the water is worked out on, which is what keeps an edge crisp without paying for it twice. Sharper ink corrects the error that carrying it makes each step, so a thin filament stays thin for a lot longer instead of quietly blurring away.