The Select Brush is used to select faces using brush strokes. Strength has no effect. Once faces are selected, you can switch to another tool for modifying the selected faces.
Hold SHIFT to deselect during a stroke and press ESC to clear the current selection.
The Select Brush also has a special right-click context menu feature that allows you to create a group from the selected faces. This is useful if you want to focus your sculpting on a smaller area without affecting the surrounding mesh, similar to masking.
When using this masking technique, be sure that 'Lock Boundary' is enabled in the Brush Settings window. This prevents the border vertices from moving.
If you want to remove the mask (group), simply explode the group back into the original surrounding mesh using the SketchUp explode feature.