There are a lot of interesting comments here, and I’ll see if I can make time to respond to some of them in another post.
I’d like to focus on the meaning of
tōng (通) in response to the OP.
My standard disclaimer: My comments are based on my instructional traditions, experiences, and research. I make no claim that these comments apply to any other branch of taijiquan, and more specifically, only apply to my own understanding and practice.
For us,
tong is much more than just power coming from the back.
Modern dictionaries definitions and usages help us get some denotations and connotations of the term. Of course, being Chinese, there are layers of meaning in usage.
tōng Verb, to open, to go through; to open up by or clear out by poking or jabbing; to lead to, to go to; to connect, to communicate; to notify, to tell; to understand, to know; Noun, authority, expert; Adj., logical, coherent; general, common; all, whole.
The definitions I think most related to taijiquan are, to open, to go through and to connect, to communicate, although leading, clearing out, and understanding may have some implications too.
Esoterically,
tong is an essential part of taijiquan instruction. There are lots of nouns used in talking about taijiquan, jin, qi, shen, yi, and so on.
Tong is a verb describing a specific type of activation.
Esoteric usage: To go through: enter, pass through, and exit, as a pipe or hollow tube would allow. In energy and force, go through and exit, as through a conduit. To get internal power, otherwise to get movement.
Tong is activation.
Tong governs your thought processes. "To go through" governs your action processes. It's the opposite of stagnation.
Tong is associated with energy alignment and energy flow. "Flow" is a mistranslation of
tong, however it is useful verbiage in English to get a kinesthetic feeling of activation. "Flow" is not flowing transitions from position to position. This meaning of flow is longitudinal (zòng, 纵) through the limbs and torso, as water through a pipe. It leads us to be able to sense our kinetic (force transfer) structure and later sense and manipulate our opponent's feet on the ground and his entire structure.
The old dojo trick the Unbendable Arm illustrates an effect of
tong. However, at a beginning level, the technique is a trick. It is set up, so you can learn the technique. If your partner didn’t allow you, it wouldn’t work. But later on, with training, you become so good that your partner doesn’t know you already did it. At that time, it becomes a useful skill. As soon we contact, he already has lost.
Tongbeiquan, 通背拳, through the back boxing, is sometimes called tongbiquan, 通臂拳, through the arm boxing. I have thought that a good metaphor for skill development. You start with an arm, progress to an "unbendable" shoulder joint, shoulder girdle, and back. Of course, the rest of your body is similarly activated, and relaxation is key to allow “flow through” and exit.
I hope this is helpful.