Because Yin and Yang depend on each other (each exists as a precondition for the other), pure Yin does not last, and pure Yang does not endure. Hence, in martial arts, there is no absolute relaxation or tension. Complete relaxation is slackness, and complete tension is stiffness. There is relaxation within tension, and tension within relaxation, the transformations of Yin and Yang are infinitely profound.
Why standing post (Zhan Zhuang) can make bone marrow full and body strong? Secret is in stretching tendons and pulling bones! Post deep in ground, very stable, no matter wind or rain, it not move! Big sky and universe, stand alone, look down on everything! Stand between heaven and earth, fill the gaps of universe! Standing post is way to make body's qi channels fully stretch. It let essence, energy, spirit go back into marrow. People who do standing post often have strong bones and tendons. One sign of this strength is person look not fat, but weight is not light, this show body healthy. Some people look fat, but is soft fat, lots of flesh, when weigh, very light, this actually show essence not stay in bones.