To align an existing workplane with an inclined face on your model:
The workplane moves to its new position.
The direction of the X-axis is changed so that it lies on the inclined face.
PowerShape makes its best guess as to how to align the other two axes (changing them as little as possible).
The workplane is twisted around its X-axis so that the Y-axis lies on the inclined face.
The workplane is now aligned with the inclined face on the model. You may now work directly on the inclined face of your model.