I just sit on the zafu thinking about this stuff. Except when I’m not thinking.
The motion of the sacrum is how I sit the lotus; I don’t put my mind in my left palm the way Suzuki advocated, yet I do find my mind where my little fingers touch the abdomen a lot. And the reciprocal of that, which is in the stretches around the sacrum. Those stretches do enter into the length of the breath in or out, now and then. That’s all I know.