This past Monday in his advanced class Grandmaster demonstrated how we can open and relax our hip by drawing in the corresponding bottom rib. He had us take a deep Du Ma (Cat Stance), and then pull back and down the bottom rib corresponding to the Yin (bent, unweighted) knee, yielding back from a sword tip advanced by our partner. He pointed out that this pulling in and dropping of the rib allows the Qua point in the groin and the tailbone to simultaneously relax and drop down, so that the bent knee can effortlessly be lifted above the waist and a powerful circular flow of energy opens between the full and empty legs.
As I practiced this compressing (concentric) activation of Yin-side core muscles later at home, I realized that a simultaneous conscious lengthening (eccentric activation) of the muscles along the side of the spine and ventral core muscles corresponding to the Yang (standing) leg stabilized the stance and brought a sense of relaxed, expansive energy. (I focused on lengthening the area between my bottom rib and pelvic bone on the standing-leg side.) I then practiced various open-hand and weapon forms, experimenting with alternating yin/yang concentric/eccentric activation of core muscles as my weight shifted from right to left, and was delighted to experience a new sense of relaxed, flowing, energetic expansiveness and stability in my movements. I invite you to try this as you practice. Ask me to demonstrate for you if my description here isn't clear.
Happy Practice!