Usage:

To use the cavity passes you have to first render a cavity pass by going to the N-Panel>A-Tools>Cavity Pass

The Panel is quite simple and to the point.

Distance controls the spread.

You can preview the Cavity pass by clicking the preview button.

Note that this is not the actual cavity pass but a low contrast preview of it. The actual pass is processed to add more contrast.

Once you are happy with the settings just click the render button and an image with the processed cavity pass will be added to the compositor node tree.

You can also hold down CTRL while clicking the Render option to automatically set up a cavity pass group node that allows you to control the edge effects(brightness and colors)