This example shows how to use the Comb SOP to control the direction of point normals by interactively "painting" over the normals.
Two Comb SOPs are used to comb the normals on a grid in different directions. A Sequence Blend SOP blends between the two so that the normals look like they are swaying.
A simple line geometry is attached to those points.
The Comb SOP is a great way to animate things like hair and grass.