This is a simple skill-building drill I just made up. It involves using the exact same brush, and only freehand drawing (no other tools), to carve out recognizable versions of predefined shapes.
Paint complex shapes faster
Add to muscle memory of painting patterns
Deconstruct patterns within shapes better and faster.
Increase stroke economy (reduce the number of strokes needed to define a given shape)
You can observe several of the key factors in this 'paint-cutting' process just by watching the animation -- for example, the need to avoid damaging surrounding pixels. This is like a somewhat hard-mode version of the problem of painting a floor without getting stuck in a corner.
Painted using GrafX2, for the clarity of hard-edged pixels.
The second version is just a faster version, because it looks good :)
There is a variation of the exercise that you can do if your paint program has a specific 'eraser' toggle, like MyPaint (which temporarily alters the current brush to paint in Erase mode rather than Normal mode). It's especially useful for working with layers.