The Stroke Width Tool lets you edit the pressure profile of curves created with line tools, typically the Pencil Tool, but also the Pen Tool and Vector Brush Tool.
The stroke width at any position along a drawn stroke can be adjusted by creating placed pressure points. At these points you can adjust stroke widths symmetrically using point handles to form a custom pressure profile that runs the length of the curve.
The benefits include:
The pressure profile is traditionally represented on the Stroke panel’s Pressure chart. The points on the chart are the same pressure points you add by clicking on the curve with the Stroke Width Tool active. Conversely, if you add a point to the Pressure chart, it will appear on the curve in a position relative to that on the chart.
The tool's context toolbar hosts several snapping controls that snaps pressure points onto existing nodes, snap to existing stroke widths on the curve or snaps the adjusted stroke width to the current or other curve's geometry.
Vector brush strokes can have their pressure profile edited just like with strokes created with the Pencil and Pen tools. It's important to understand that vector brush strokes have Size Variance and an Opacity Variance settings that control how much the stroke size and opacity responds to pressure. If you edit the pressure profile with the Stroke Width Tool you may see size and/or opacity changing because of these settings.
The point’s handles extend symmetrically outwards from the ‘on-curve’ point to the current stroke width.