Freestyle > Adding Detail to the Control Mesh > Splitting Mesh Elements > About Splitting Mesh Elements
  
About Splitting Mesh Elements
You can split one or more faces or edges on the control mesh into multiple faces or edges, thus creating new Freestyle surfaces. When you split a face, the face is split into a number of faces, where the number is calculated by adding one to the total number of edges in the selected face. When you split an edge, the split propagates through the quad faces in a loop and ends before it runs into a non-quad face.