Even in the most "simplistic" of stances, even slight foot rotation will (either) aid, or render the legs useless in achieving the desired motion.
The foot positions shown above (in the “stances” examples) illustrate the correct positioning of the tori's feet. Though (initially) seeming odd, that positioning effects both (though mainly) the legs, and the torso's directional options (and thus, The most efficient direction for the application of a technique). The user's positioning of the feet will determine how much of the available force can/will be directed into an application in a given direction.
Though no numbers/percentages are provided, the diagram is only intend to illustrate the differences between the positional situations shown.