From Synfig Studio :: Documentation
- It is possible to paint on the Image Layer
- Basic pressure sensitivity is supported.
- Image size is automatically expanded when you paint outside of image boundaries.
- An Image Layer is automatically created if the current layer is'nt an Image Layer.
- Eraser check box. When checked, the brush acts as a rubber.
- Display the set of configured brushes (MyPaint brushes format). By default, Synfig is installed with a selected set of brushes from "Concept Design (C_D)" pack by Ramon Miranda.
From the Preferences Dialog, you can configure the "Brush Presets Path" to use other brushes package (for example the ones listed by MyPaint)
Other sources of information
here links to cool synfig video brush usage and frame by frame animations