What is Muller-Breslau's principle for drawing influence lines?

Muller-Breslau's principle states that the influence line for any response function equals the deflected shape of the structure when a unit displacement is introduced in the direction of that response. For reactions, remove the support and apply unit displacement; for moment, insert a hinge and apply unit rotation; for shear, cut the beam and apply unit vertical displacement. This principle simplifies drawing influence lines for indeterminate structures.