Zhan Nian Lian Sui is an important concept in Tàijíquán practice. Its four words (Adhere, Stick Up, Connect, Follow) are an admonition to what is a correct approach for close-quarter application. Performing Zhan Nian Lian Sui depends both on mental and physical conditioning.
More detailed information on the songs of push hands can be found YCGF North America: The Song of Push Hands.
The four words refers to two pairs: Zhan-nian and Liansui. Zhan-nian jìn has to do with offense, while Liansui jìn has to do with defense. Jìn means cultivated skill or power or ability — a type of trained force. In this case, we consider four abilities in two pairs that transform our approach to grappling and allow us to develop higher-level approaches in our Tàijíquán. Expressing Zhan-nian jìn and Lian-sui jìn correctly is an important prerequisite for developing Dǒng jìn, the proper understanding of cultivated force. These skills all come from relaxation and developing a relaxed feeling.
Once you have cultivated relaxation, you build up these skills from that foundation. But relaxation is not the common external view of that idea. In application, you need express force. However, in doing so, you should stay relaxed, and then in an instant when you need to release force, you integrate your force together. Otherwise, you can’t do anything. However, you are constantly storing as you release, not storing then releasing. Opening and closing, not opening and then closing. This is a core Tàijí principle.
In training, you need the prerequisite foundation of cultivating a relaxed state, but you also need to cultivate specific body condition or qualities and know the correct way of doing things, for example details on posture like the nine key points, so that you are able to integrate your force.
This develops the physical organization of your body so you can be relaxed and stable enough to get to the point of understanding Zhan Nian Lian Sui. Then you can achieve the state of Dǒng Jìn (understanding force) and develop what is called a Tàijí feeling that permeates your practice.
Taoism leads to the Tàijí idea, leads to Tàijí principles, leads to qualities we want to cultivate, leads to their physical expression and our physical change. That is a hierarchy that lets us understand what to do, but in training we need to work from the bottom-up from physical movement, to qualities, to principles, and finally the idea of what is Tàijí.
Some things we always work to improve, such as our ability to relax and sense (sensitivity). No matter how good you are, these can continue to improve, so we always need to work on basics and strive to improve.
Form (Ti) and Application (Yòng) together will yield good health. Once you know the principles and ideas, Tàijíquán will change your thinking. Most time is spent on form, and a smaller amount of time spent on fixed push hands. Then freestyle pushing, which is the focus of application (yòng). Then special training for combat.
Form makes the body good enough for application but today there are too many different Tàijíquán forms. For example in Wu style, the Hong Kong, Shanghai, and Beijing groups all practice form differently. There are also too many styles (i.e., Yang, Hao, Wu, Chen, CMC “Professor” style, Sun, etc.). Historically, individual lines of practice became famous because of the high level of skill of a master practitioner. But just because a person trains with such a person there is no guarantee they themselves will become any good. Usually only a few people will become excellent practitioners in a generation. For example, a person might practice a form or training routine, but maybe the fine details of their practice are not correct.
There is not very much fighting between traditional martial artists these days, so people who train a long time do not get direct feedback on their skill and wind up not knowing their actual level of skill. This is a big problem for traditional martial arts groups. People can go for a long time without realizing they are making an important mistake, in contrast to arts that have a competitive aspect.
It can be the case that some, maybe even most, students and disciples of different styles (regardless of style) may not train in a way that will lead to the high-level skill that made Tàijíquán famous. For example, if they are taught something and do not properly understand it, they may practice incorrectly for a very long period of time, especially if they train at a distance from a teacher. It is not very helpful to compare styles directly (Yang vs. Wu vs. Chen) in terms of their training routines, since these challenges affect all lineages, but it is still possible to evaluate individuals to get a sense if they understand Tàijíquán or not.
It is hard to comment on a person's Tàijíquán skill without putting hands on them directly, but you can tell if basic things are wrong with their practice through observation — if so, you can conclude that they likely cannot express high-level Tàijí skills using their approach. This is because without proper basics, they will not be able to develop over time the sense of relaxation and sensitivity required to integrate force properly. If someone practices Wu style, and does not have the nine key points of posture, we know the style will not work for them, as their internal Qì will not move correctly. By watching for those nine key points and not finding them, you can know they probably cannot execute high-level skills. This is just one example specific to a style I practice.
