Investigations on Pi-cell, a sandwich cell with the director rotating by 180°, are demonstrating the possibility to obtain nematic transitions between two textures with different topologies, for instance between an untwisted state and a Pi-twisted one. Fast topological changes can be observed by means of suitable electric fields or by micro/nano-confinement in the nematic bulk or close to a boundary surface. A strong frustration, in fact, as in the case of a strong local distortion, can induce transient and/or local biaxial states inside a nematic calamitic phase and the resultant nematic dynamics demands a full Landau-de Gennes Q-tensor description. In this work, we present Q-tensor numerical models capable to describe the nematic order dynamics in full details, comparing theoretical results with experiments. We will also discuss as nematic order reconstruction close to a boundary surface with strong or infinite anchoring conditions can provide transitions equivalent to anchoring breaking.