The one constant in the practice of Tai Chi and playing push hands is change. If you are caught in a fixed position then you will end up resisting.
This is where the importance of knowing the form thoroughly comes in. If you really have learned the form well and practiced it to the point where you are free of it then you will always be able to find a position of change. This is in accordance to the old saying that if you are willing to give in to others, wherever you face will have no peer.
This is not to say that you are invincible and invulnerable to knives and spears. It is merely stating that if you can find a way to change then an opponent will have a hard time finding your centre of gravity and be unable to control your balance. So if they cannot control your balance then they will have a hard time trying to defeat you.
During Easter recently I spent time with a student watching some videos. I showed him Artisans Reboot particularly the episodes on mee sua, pot making and rattan furniture making. These three episodes were the best in the entire series and have good lessons for those interested in learning a skill. For the Tai Chi learner it was interesting to observe body movements and their relationship to the principles of Tai Chi.
My student had no problem catching the things I pointed out since he has spent enough years learning Tai Chi to have a certain degree of understanding. The important thing now is that he be able to incorporate them into his movements as if its the natural thing to do. Then his kung fu, the skill rather than the style, will be good.
A reader might ask how to naturalize the skill. As I would tell my students use the form to train the skills. At this juncture some might wonder if this is true as they do not feel any significant difference in their skills after practicing the form for years.
The root cause here is that the student is just going through the motions rather than actively practicing the skills. Waving your hands about is not doing Tai Chi. Doing Tai Chi means to ensure that every movement you make contains a number of principles within them. These principles are brought alive using intention and are compatible with physics. The movements are also multi-layered, containing different learning principles at different times in your learning journey. This means that at different times in your learning the way you do your form should be different. If it is not, then you are either playing a form devoid of skills learning, you are taught a fixed way or you have not practiced enough to make significant progress.
You must then keep practicing the form, constantly seeking to fill in the details until every movement you make has a reason behind it and the movements are interrelated in a way consistent with the principles. This is called knowing the rules.
When you know the rules then your teacher must now teach you to bend the rules. This means you know learn how each movement can be practiced in different ways by varying factors such as timing, angle, speed, strategy, etc. For example, I would teach beginners how to do only one version of Step-Up, Parry, Punch. An intermediate level student would learn how to do up to three versions of the technique. An advanced student would then take the three versions and practice them in a way that an outsider would not be able to discern easily which version he is expressing. The advanced student would also learn how to issue power for each of the three versions of the technique.
The next stage is then how to break and remake the rules. The student at this level would learn the core principles that are always present in each and every movement. Actually, this information has already been taught to beginners but at that stage they would not realize the significance of the teaching. The advanced student would understand the importance and significance of complying to physics in their execution of the techniques because this essentially is akin to fulfilling the principles of Tai Chi Chuan at a certain level.
After going through the three stages the student can now play the form in many ways. You can now say that the student truly embodies change in his playing of the form. By highlighting a principle more, changing the strategy or issuing a certain type of power he can change the way he plays his form. Tai Chi to him is just Tai Chi rather than X style Tai Chi, Y style Tai Chi or Z style Tai Chi. Yet when he has to make a certain point he can then express certain principles more to render his expression as X style, Y style or Z style.
There is still more to the journey of the student but the above is what is required to master Tai Chi.