Baecklund transformations, energy shift and the plane wave limit

We discuss basic properties of the Baecklund transformations for the classical string in AdS space in the context of the null-surface perturbation theory. We explain the relation between the Baecklund transformations and the energy shift of the dual field theory state. We show that the Baecklund transformations can be represented as a finite-time evolution generated by a special linear combination of the Pohlmeyer charges. This is a manifestation of the general property of Baecklund transformations known as spectrality. We also discuss the plane wave limit.