This paper presents the results of studies regarding the effect of nonconventional plastic deformation on the mechanism of plastic flow of metals. The applied method, called KOBO extrusion, in which monotonic large deformation is accompanied by cyclic reversible plastic twisting, falls into the category of severe plastic deformation. The cyclic reversible plastic twisting, which proved to be redundant in terms of changes in the material's geometry, appears to be a particularly efficient source of point defects. The experimentally proven Newtonian viscosity of metals at low temperatures under KOBO extrusion conditions implies that it is necessary to conduct a study on the crystal plasticity from the point of view of the constitutive law in which a crystal's viscosity totally controls the kinetics of the process. The well known dependency of the viscosity parameter on the diffusion coefficient, which, in turn, is a function of point defect concentration, dictates the link between the frequency of reversible plastic twist and the crystal's viscosity and, consequently, determines the mode of plastic flow. The results of this work provide experimental verification which justifies this new approach to crystal plasticity.