The lasso brush combines the lasso selection and paint bucket tools.

As you draw a shape with this, the area will be filled with color. A straight line is drawn to close the shape between where you started and the current position of the cursor. Lifting your pen will complete the shape.

In the settings panel you can set the fill to be solid or patterned.

Setting the lasso file mode to erase will erase an area as you draw it out.

Setting the tool to auto-erase activates the lasso fill in erase mode when you press and hold the shortcut for your eraser