In my experience, the lotus is about isolating the motion of the sacrum. I like John Upledger’s explanation of that motion, based on his own research and the theories of cranial-sacral osteopathy: the skull bones, the spine, and the sacrum flex and extend with changes in the fluid volume of the dural fluid (ten cycles a minute, Upledger says here: An Interview with Dr. John Upledger, D.O., O.M.M.).
Isolating the motion of the sacrum is important, because the fundamental postural activity is generated by the stretch of the ligaments that connect the sacrum to the pelvis. There are ligaments between the sacrum and the wings of the pelvis, ligaments between the sacrum and the sit-bones (the sacrospinous ligaments), and ligaments between the sacrum and the front underside of the pelvis on the left and on the right (the sacro-tuberous ligaments). Stretched ligaments can generate muscular activity to relieve their stretch, without any exercise of volition; when the ligaments are paired there’s a phenomena called “reciprocal innervation” that arises as ligaments alternate stretch and activity from side to side.
The way to generate feeling for stretch and activity is to attend to the place of occurrence of consciousness, as necessary to realize the breath in or the breath out, and relax.