This is the requirement of the erected calf. In many styles of martial art, this concept is referred to as zhuang, pile, stake or zhan zhuang.
This photo graph is designed to deal with knee problems in training.
In order to avoid injuries, the most important thing to avoid is the knee pushing outward away from the body, as illustrated in the picture above: the yellow dot cannot move towards the blue line. The blue line is an imagined wall that the knee cannot push against.
- Question: What about the principle of the knee going up and down?
The knee must act like a ball, it aims up and down but it does not physically move up and down at all. The aiming of the knee, one up and the other down, is called “intent” for the knee.
- Question: What about the concept of the knee not exceeding the foot?
In general, that principle is a correct one. But as a matter of principle, it is more important to ensure that the kua pushes down towards the knee and the knee pushes towards the foot (floor). This principle ensures that the knee does not have a horizontal orientation, thus does not move sideways outside of the foot.
The following youtube clip shows the actions of the rear knee.