1. All primary
concerns are: The manipulations of forces within you. You asked how it relates in a classical sense? This is covered in so many sayings, descriptions and models it is hardly worth mentioning. All point to the primary Motion in stillness, as the first level. the most important level as it sustains you and balances you and manipulates before contact. Hence solo work.
How it relates martially?
2. All secondary
concerns are: the manipulation of force enacted upon you
a. You can join centers and manipulate force from outside within you. This is in my view late and reactive and can be played with. I also do not agree that it is a needed step. It implies -too all I have met who use it-,a passive body that is receiving. In and of itself this is not a method in keeping with any discussion I have had with Master class teachers, or have read of many others. Moreover, in a fighting sense, it is inferior.
b. You can manipulate forces within you that are continuously resolving yin and yang. Thus any force enacted upon you meets force in motion and action, thus reducing and redirecting upon contact. There was never an intent to make a connection, and there is a greatly reduced chance of playing your center. This can easily transfer from internal to external with a smooth transition and no break in either feel or load. There are various modes to accomplish this and not all agree.
I do not agree in the least bit that the best way to accomplish #2 The manipulation of force, is by passively connecting and then attempting connection to make kuzushi. While we agree that fighting is an ancillary or parallel issue, the deliverable results in that venue are in fact superior to the model of joining centers to move. Internal movement prior to motion accomplishes significant division and manipulation of initial force... upon... you without much thought as it is a constantly trained state. Thus #1 is faster, and harder to play with from the onset.
Ask LCD and then WHJ where a jin force starts. You might be surprised to see they do not agree. Then you can read certain amateurs stating it is THIS and always was THIS.
There any number of teachings and discussions involving Chen Yu, CXW, LCD, LCG, HJS, his ideas from CFK and Feng's, which do not all match and compare them to LDX or HJB and go on and on from there. DR has it's own basis of kuzushi in it's own model describing how motion precedes receiving. Now, add the internal non-visible component to that model and you have yin and yang present, this is expressed by Ueshiba as well in his discussion of heaven/earth/man "releasing the mountain echo" and ask why that is, then on to various chi gongs to move energy within as a constant state..... prior to and during actual
When they contact motion in you, they are automatically reacting even when they don't realize it. And no...I am not talking about moving around...