My experience has been that focusing on the Kua is not particularly practical in application or in development of combat skill. Yes, the lower body and torso should be well trained in strength and coordination, but, in my experience, the main point where power is lost on contact of the arm is at the shoulder. If one uses the whole body to support the rotation of the shoulder joint, what you have is immense power expressed through the arms. If one uses their whole-body to support their hip rotation it makes for very powerful kicks. It is very useful to focus your entire mass on supporting the movements of the limbs, on the rotation of the joints.
https://www.youtube.com/watch?v=jFSsLfP95tQ&t=274s
How is Taiji different from what the above video shows? Namely, the precise control of the joints with whole body support, not the whole- body moving as if one piece.
I have found the idea that in-order to move my hand I have to move my waist or rotate my hips or do this or that step and stance, to all be unnecessary complications. Rotating the joints, extending and contracting the limps while supporting them with my whole-body mass, not by movement, but by relation, seems very effective.