Can someone please explain to me how it works? I'd really like to play with this but I don't understand what's happening, and why my brush only seems to make things happen sometimes and doesn't seem to reflect what the image representing it does.
Sure. There're two columns of buttons - left one represents brush type, right one represents brush size.
Brush size one is obvious (once named), the brush type column is a little less so.
Here's a qualitative description of what they do
Q - White brush
W - Average dither brush - replaces an area with a solid dithered pattern
E - Automata brush - usually produces noise which can be grown from any area of mixed colour, produces more intricate patterns when used by clicking singly on existing patterned/geometrically simple areas.
R - 'Square' brush. Makes things darker, tends to create square edges
T - 'Grow' brush - grows black areas.
Y - Black brush
(I think the T one is maybe the most useless of the lot? I might replace it with something else...)
I think the main way to actually draw with this is to block out areas in black/white, then to use the other brushes to smudge/squidge things around.