I like images
a lot of it had to do with the psoas and the positioning of the hips - you're familiar with the wing chun horse stance, that one purposefully has a tight...tight isnt really the right word...the position of everything simply induces a bit of tension on the psoas, a cousin of full lotus, from a certain point of view. any time you root are are quasi locking yourself to the ground, and probably the closest & most significant muscle aside from the long femoral ones with regard to the "hinge-point" is the psoas, so that bit of tension even if induced from lengthening/twisting of the psoas helps "seat the hinge" whereby you may efficiently transfer force - the application utilizes the kidney sinew channel more.
[/img]
I like this cross section - you can see just how central the psoas is
and the point of attachment on the femur for the psoas
so by changing the angle of the pelvis as a whole, a lot of what you're doing is either stabilizing or relaxing the psoas muscle - the tenderloin, the kua, if you were to have to point to a single representative structure.
stand there in the wing chun horse stance, then run your finger along the inguinal crease and you should be able to feel the psoas there, when tense you should be able to feel the muscle a bit flexed.
now change the angle of your pelvis so that the lumbosacral junction is nice and flat (i.e. parallel to the ground) - when balanced properly you are able to relax the psoas completely! mess around with feeling your psoas flex & relax with your hand on the inguinal crease and you'll get a sense of...like standing a pencil up in its eraser, when everything is properly positioned over center then there is much less need to stabilize one direction or another.
with relaxation comes more qi flow - which is HUGE especially when speaking of such an energetically significant one such as the psoas muscle - with more flow, one is able to be better grounded - thus the relaxed method equates to a more "grounding" focus and the "ready-set" method of a stance like the wc horse has much more of a rooting focus. simply by changing the angle of the pelvis a bit changes the energetic focus of the stance. you can think of the relaxed method, not utilizing extras like the kd sinew channel quite so much, as utilizing the kd primary more - especially since more relaxation, more flow, the psoas flows right along with the kd channel.