In this case Jonny uses the Viewport Canvas to paint interactively the rotation map. Each color (from black to gray) defines a degree or rotation (from Min% to Max% in the range parameters). For example, if you paint using a gray color (128/128/128) and the rotation range is 0º / 359º, the items will be aligned with a rotation of 180º.
You also must take in account that each component RGB, defines one of the directions XYZ. So, to rotate in Z, it is necessary to convert the grayscale map to a blue range. This can be easily done using a Color Correction textmap, as shown in the screenshots of the documentation.
Basically, the rotation map is a map of normal vectors. All techniques about normal maps are applicable here.