Surface Mode:

1. Under Modify and Surface Mode, select a plane which you would like the edge to fix in and press Space to lock it.
2. Under Modify and Vertex Mode, Hold and Drag the vertex on 3D view.
3. When Lock Mode is off (default). The vertex can be only moved on the locked plane.
4. When Lock Mode is on. The vertex can be only moved on the co-planar.