Comparative pilot refining trials with Norway spruce chips were conducted in order to gain a better understanding of the principal mechanisms on fibre level for achieving radical energy reduction in the novel ATMP refining concept. The ATMP pulping configurations proved superior energy efficiency in pulp quality development, compared to TMP, RTS and F-RTS concepts. Development of high tensile strength did not progress at the expense of light scattering, even with process chemicals present. Extensive testing of individual particles in long fibre, middle and fines fractions in combination with analyses of properties of fraction sheets has shown that Impressafiner/Fiberizer pre-treatment opens the wood structure, allowing for early development of desired single fibre properties and generation of fibrillar fines. Peroxide combined with magnesium hydroxide affected the interior of the fibre walls, thereby facilitating improved fibre flexibility, collapsibility and swelling. Bisulphite mainly affected the fibre length distribution, reducing the coarse fraction R14 and promoting the generation of considerable amounts of coarse fines and middle fraction particles.