What is "Set normals from faces"? If you are in the mesh Edit mode, you can find a button called Set normals from faces in the Tool Shelf, Shading tab, Normals section. The Blender manual seems to have no information about this feature (or at least I can't find any).

What does it do and in which cases could it help? I would like more information than the brief message from the tooltip.
You need to have Autosmooth activated. Then you can select a few faces, and with a click at the tool the normals of the involved vertices will be calculated in direction of the normals of the selected faces. Where two faces meets each other the normal will be middled.