The process (connecting the upper thighs and the perineum) is difficult to describe, because the fundamental is the occurrence of consciousness, and how the occurrence of consciousness resounds in the stretch of the fascia. The lion is totally relaxed, and enters into motion totally relaxed, because the action comes out of feeling generated by the impact of consciousness on stretch. The lion does not intentionally stretch, yet the presence of stretch is evident in the relaxed, tenacious movement of the lion.
The top of the thighs and the perineum, myself I look for the stretch from the sacrum to the sit-bones to generate activity in the obturators, which rocks the pelvis side to side on the hip bones. The obturators run under the pelvis, and can lift the pelvis off the hips slightly as they contract. I look for stretch in the sacro-tuberous ligaments running diagonally from the sacrum to the lower front of the pelvis, and activity in the piriformis from the upper legs to the sacrum as a result. There is also activity in the PC as a result of this, and likewise as a result of the pivot of the sacrum on the wings of the pelvis. Fundamentally the place of occurrence of consciousness and the stretch occasioned by the place of occurrence develop feeling.
I confess my mind is not in the tan-tien that much. I do practice a lot with sensing the coordination of the motion of the sacrum in the muscles of the abdomen where my little fingers touch, doing the mudra of soto zen; side to side, diagonally from the thighs, and up from the pelvis, both little fingers on the lower abdomen.