The plastic flow of austenitic steels can be disturbed by instabilities of different kinds. At the temperature close to absolute zero, regular stress drops at constant strains occur, which identify the characteristic phenomenon of intermittent plastic flow (IPF). A single serration of the process includes four stages: elastic strain, plastic flow with a pronounced hardening, rapid stress decrease, and stress relaxation. In-depth analysis of systematic investigations on microstructure and deformation fields reveals the underlying mechanism [1]. An energetically favorable orientation of austenite - Brass (B) controls the emission of the macro-shear bands during tension along or transverse to the rolling direction. The combination of profilometric examinations, both with 3.512 um and 0.756 um steps, with the synchrotron X-ray diffraction and electron backscattered diffraction (EBSD) studies shows that the rapid displacements proceed along four planes {1 1 1} of two variants of the orientation B [1]. In this way, a zone of sample contraction associated with the serration is formed. Each of the four macro-shear bands is built of micro-shear bands running along the twin boundaries contained in grains B. The analysis of geometrically necessary dislocations (GNDs) shows their consistent piling up on the barriers formed by inactive twins of grains B oriented perpendicularly to the tension direction. The overcoming these blockages results in rapid micro-shearing, leading to martensitic transformation. Due to strong physical conditioning, the discovered mechanism of unstable plastic flow remains valid at room temperature and controls the propagation of localized plastic strain along the loading direction, which delays the fracture [2]. At room temperature, the rapid shearing is replaced by a continuous process of creating chains of grains B. Inside them, martensite embeds coherently, which enables the stable propagation of the zone of concentrated plastic strains. The discovered mechanism is described by an analytical, symmetry-based model that joins elastic and plastic deformation.