Select one of three initial planes (XY, XZ, YZ) which is
located in the middle of the of scene in the Operation. A plane is approximated by a rectangle which crosses the whole bonding box of the complete scene.
Use Crl+Arrows keys to move the plane in the position required.
Use X,Y,Z keys to increase the plane in the corresponding dimension and Shift-X, Shift-XY Shift-Z to decrease it.
Press "Enter" to execute the operation or Esc to abandon it.
The results of the operation are presented in the new 2D-Viewer window.