I thought my experience would translate into progress at the lotus, but after a lot of years, I decided I would have to figure it out, instead.
So here’s the story: as Carl Bielefeldt translated a master’s words in the first edition of Dogen’s Meditation Manuals, “seated meditation is not holding still”. The fascia and ligaments of the body can generate muscular activity, involuntarily, if they are stretched sufficiently. Since the muscles and fascia are basically in pairs, the action of posture is fundamentally the reciprocal innervation of muscular tissue as the stretch of fascia alternates from side to side.
The basic stretches you are interested in for the lotus are the stretches of the fascia that connects the sacrum to the pelvis; the sacrum moves, forward and backward, side to side, and around with the changes in volume of the fluid in the dural sac, surrounding the brain and the spinal cord all the way down to the sacrum. The fascia that connect the sacrum to the sit bones stretches, and generates activity side to side; the fascia that connects the sacrum to the forward undersides of the pelvis generates swivel activity left and right; and the fascia that connects the sacrum to the wings of the pelvis stretches and generates motion forward and back.
Watch the motion of the sacrum, relax and look for feeling as the location of the occurrence of consciousness leads the motion of the body, in the legs and throughout the body. The presence of feeling is the correct alignment of the spine, and you will have to open a bit to pleasant and unpleasant feelings with the occurrence of consciousness in order to realize the stretch that generates the activity of the lotus.