To investigate the mechanism of grain refinement during severe plastic deformation at cryogenic temperatures, the helical channel angular pressing (HCAP) process was employed, and the grain refinement behaviour under twin- and dislocation-mediated plastic deformation modes was examined. Cryogenic HCAP (cryo-HCAP) experiments were carried out at −196 and −76 °C on C18150 copper alloy and 6005 aluminium alloy, respectively. Microstructural evolution in both alloys was characterised using EBSD and TEM, and the results were compared with those obtained under room temperature processing. A geometric dynamic shear grain refinement mechanism is proposed to elucidate the observed behaviour in the cryo-HCAP process, based on the formation of serrated subgrain boundaries. At cryogenic temperatures, when the equivalent strain reaches 3–5, twin bundles or deformed grains are thinned to approximately 1–3 times the subgrain diameter, and torsional deformation occurs. In the twinning-dominated plastic deformation mode, twin bundles oriented perpendicular to serrated twin boundaries are sheared off at twin gap intersections, leading to grain refinement and the formation of deflected, chain-like twin bundles within the matrix. In the dislocation-dominated deformation mode, equiaxed refined grains emerge when the deformed grains are pinched off perpendicular to the serrated subgrain boundaries. Under the combined effects of cryogenic temperature, severe plastic deformation and torsional strain, both deformation modes share the following characteristics: when serrated (sub)grain boundary structures and critical thinning dimensions are achieved, equiaxed refined grains are produced. Furthermore, the fraction of high-angle grain boundaries increases as temperature decreases.