Menu: Geometry->Cut 2D polygonal
Here you can define a polygonal on the screen. This polygonal is then used to cut the triangular and quadrilateral meshes to create a cut mesh of lines. So when the surfaces are deformed, the polygonal cut is deformed too. And vice versa, if a polygonal cut is defined while the surfaces are deformed, when turning them into its original shape, the polygonal cut is "undeformed" with them. These cuts can be imported and exported and are saved between sessions, like normal cut planes. Quadratic elements are handled as linear ones, like normal cut planes. Nodal results and 1gp results are passed to the new cuts as well.
Note: pressing the Alt key while selectinf the second or third point will tie the dynamic line horizontally, vertically or aT 45º angle.