Fajin simply means 'Refined Force'. The question is, What is being 'refined'?
Training progresses from big movement, to small movement, to 'no movement'. The tendons must become strong and supple. Relaxation must be present in order to 'sink the chi' and for the bones to play their proper role. Fajin is the natural product of awareness and very specific internal conditioning.
Understanding the structure leads to relaxation, which leads to understanding the energy (chi). There are no shortcuts. To cultivate 'the energy' without a deep understanding of structure and relaxation, leads to 'artificial power'. Artificial power is easily defeated by natural power. This arises from the differences of the quality of the awareness produced by the differing training approaches.