On Sequential Movement...

The transmission of internal force in Taijiquan adheres to an integrated system governed by the guiding principle: "From the root segment to the middle  segment, and subsequently from the middle segment to the distal segment.”  The core principle lies in "connecting each joint in succession” in an "unbroken continuity through every segment节节贯串“.

Many practitioners perform sequences with stilted disconnected transitions,  failing to attend to the sequential continuity of qi and internal force.  Yet in every single movement—every raise of the hand or every step taken —one must embody the maxim: "Rooted in the feet, generated through the legs, directed  by the waist, and expressed through the hands”.  The waist acts as the governing hub for transmitting power.   Achieve complete relaxation and sinking, letting it settle fully into the feet.  The more thoroughly the whole body sinks, the greater the force the feet exert against the ground, and the stronger the ground’s reactive force. This force, travelling upward through the body, naturally becomes deeper and more substantial.  In execution there must be a persistent awareness of force originating at the feet, propagating sequentially through the legs, waist, shoulders, elbows, and wrists, until fully expressed at the fingertips in a complete, unbroken sequence.

While the issuance of force through the hands appears to be a hand technique, it is in fact the product of integrated whole-body coordination. To overcome practice plateaus, focus on three key points: systematic relaxation of the entire body (especially opening shoulders and kua), identifying the source of force (firm grounding in the feet), and maintaining unbroken continuity through every segment.  And sustained, reflective practice in order to  internalise the essence of "consecutive articulation through all segments”.

Fantun

 The concepts of liutun溜臀 and fantun泛臀 in Taijiquan and similar internal martial arts are often hard to describe or define. As soon as you try to describe or define these movements, you risk exaggerating them.  It requires the practitioner to observe, feel, and understand the subtle distinctions with great care.

Liutun 溜臀 -  "settling the buttocks” is a physical sensation that arises only when the body is fully loose and open. It happens naturally and drops into place from fangsong 放松 (letting go of unnecessary tension).   If active force is used to deliberately shape the body to look like "settling the buttocks” it is already overdone and the correct state is lost.  Settling the buttocks is not a method to practise or perform - it is the natural result of being relaxed.

Settling of the buttocks occurs when the entire body is relaxed and integrated. At this point, the head and the tailbone -  Baihui 百会 (the point at the crown of the head) and Weilu 尾闾 (the end of the spine) - operate as a single unit. The spine gently arch upward and lengthen, while the tailbone softly sinks downward. These actions create a sensation of the buttocks dropping and settling down, with the tailbone feeling as though it is gently "ploughing" forward into the ground. Another way to understand it is that the energy and weight sink down from the Jianjing 肩井 (shoulder well) points settling toward the Mingmen命门 point in the lower back/buttock area, then continuing downward through the knees until it finally roots into the feet.

Fantun 泛臀 -  expansion of the buttocks is absolutely not what many people think of as turning the bottom out.  It happens within the state of “effortlessly lifting the crown and sinking energy to the Dantian” 虚领顶劲，气沉丹田.  The left leg rotates inward clockwise, while the right leg rotates inward anti-clockwise simultaneously. Energy rises upward along the outer edges of both feet, and both kua open and expand outward. At this moment, both buttocks gently expand outward, one slightly to the left and the other to the right.  The inner dang 裆  opens up and the area of the crotch becomes rounded and full 圆裆.  Like settling, expansion of the buttocks is not a method or technique to imitate—it too is a natural result of deep relaxation.

“ Settling” the buttocks is downward sinking  and inward contracting, rooting towards the earth, thus providing foundation and stability.  "Expanding" the buttocks is upward lifting and outward bracing, enabling energy to open and extends, thus creating space and flexibility.  Both sensations exist simultaneously and are equally important and are two sides of one single relaxed state.  The difference and the correct position is only a matter of tiny degrees, requiring practitioners to carefully feel and understand the subtleties for themselves.