To draw an open line, release the mouse button when the line is the desired shape.
To draw a closed shape, hold down Alt (Windows)
or Option (Mac OS) as you drag. The Paintbrush tool displays
a small loop . Release
the mouse button (but not the Alt or Option key) when you are ready
to close the shape.
Illustrator sets down anchor points as you draw. The number of anchor points is determined by the length and complexity of the line and by the Paintbrush tolerance settings.