I first traveled to China over twenty six years ago. I traveled there with Tom Bisio and another friend Tom Clifford. Tom Bisio was at the time vice president of North American Tang Shou Tao. Tom C. and I were also representing that organization. On that first trip we spent intensive time in Beijing with Wang Shitong, Tianjin with the Gao Bagua family and Taiyuan with Master Li Guichang and some of his disciples. At the time I had been practicing Tang Shou Tao Xingyi Quan for seven years and other martial arts for many years before that. I first met Song Zhiyong during our time in Taiyuan and when he began to show us his Xingyi we were all very surprised because he seemed to move under his skin like no one we had seen before. We joked about the possibility that he might be an alien (he isn't) but it took many years to truly understand what was going on inside his body and how his skill was developed.
On this site you can find a lecture given by Master Li's disciple Cheng Quangong. Early on he asks people to explain what they think Xingyi Quan means. You can read the responses on this site but basically they just say that Xing means form or shape and Yi means intent or mind. Now this is true in a theoretical sense but it doesn't express what you should really feel in the body after a long practice of Xingyi. Cheng Quangong then quotes Master Li saying, basically, that containing in the interior is Yi and releasing to the exterior is called Xing. Now the "six harmonies" are a way way to help with understanding this. The harmonies are another theoretical guide. On the exterior the hand harmonizes with the foot, the elbow harmonizes with the knee, the shoulder harmonizes with the hip. On the interior the Shen harmonizes with the Yi, the Yi harmonizes with the Qi, the QI harmonizes with the Li. Now this is basic Xingyi theory that every student learns within the first few lessons. But the next question is what makes Xingyi Xingyi Quan. In order for it to be Quan (the fist) Master Li said that the Yi (containing in the interior) can be released externally and the Xing (releasing in the exterior) can be contained internally. Of course just reading this can be confusing but it's important to realize he is not talking about a theoretical understanding, he is talking about body knowledge or what you actually feel and develop inside the body.
During one of my trips to China after Master Li passed away I was training with my school brother Song Zhiyong and he had me place my hand on his waist. Then he did the five elements without moving the outside of his body so I could feel the changes as they happened internally. This is the Xing contained internally and without it the the body is empty. A person's forms might look very good, fast and powerful, but the torso of the body may still be empty internally. In usage Xingyi does not have to look like what we might think of as Xingyi, it is still Xingyi if the inside of the body contains the form. This does not mean however that the inside of the body somehow echoes what the outside looks like. Just visualizing a shape inside the body will not help and may instead hinder any real development.
So of course knowing this is useless if I have no method to develop the interior. Just doing the forms and learning techniques and even doing standing practice might not be enough. In many ways I did everything backwards. First I learned forms and usage etc. and only later did I begin to practice the way Song Zhiyong teaches, which is to begin by spending a long time doing Tu Na Si Ba and Santishi. In the beginning Song would only say "do Si Ba without using the Yi, just follow the Jin". This was very confusing to me because so much of the time in internal arts we are taught to visualize and use intent. What I ultimately began to understand, after learning the movements to the point that I didn't have to think about them, was that by removing any preconceived notions of what something "should" feel like my awareness began to develop to the point that I began gradually to experience new sensations deep in the body that were causing the outer movements to occur. These sensations are what we refer to as the Jin or energy. Song Zhiyong would not say what he felt doing Si Ba but he would give certain hints and if, over the years, I described a certain sensation he would indicate if I was on the right track. This way of teaching keeps the student from just trying to imagine in themselves what the teacher describes and instead encourages him/her to deepen their own awareness. If done correctly over time the internal body, the body within the body, will be doing all of the elements which later will be expressed in the outer shapes of Xingyi. When doing standing practice the body internally may spontaneously do the different elements without changing the outside form. When this is developed Xing bu Xing, Yi bu Yi and Quan bu Quan all become possible.