Mechanisms of the step permeability are studied theoretically with the use of a simple terrace-edge-kink model of atomic events at the step edge. The model takes into account the essential details of the potential relief for adatoms at the step including possible asymmetry of the relief arising from the Ehrlich-Schwoebel barrier or/and the difference in the adatom bonding at the neighboring terraces. The step permeability and incorporation coefficients are obtained as combinations of the rate constants of atomic processes at the step and the kink density. The step permeability condition is formulated in the form of the inequality on activation energies of the atomic events and the kink density, which allows to specify the role of attachment-detachment processes, edge migration and formation of kinks in the step permeability. It is found that formation of non-equilibrium kinks (the 1D islands at the step edge) can result in the non-monotonous temperature dependence of the step permeability.