The phrase “one part moves, all parts move” will be familiar to anybody who has practiced Tai Chi or one of the internal arts for a while.
Taken literally, I think you could come up with a number of bizarre interpretations for what it means, but like most of the phrases from old Chinese texts, it makes a lot more sense when you already know what it is talking about, and very little if you don’t.
There’s another line from the classics that I think relates to it:
“The whole body should be threaded together through every joint
without the slightest break.”
This is a recurring theme in old Chinese texts written about martial arts, not just Tai Chi, that the whole body is connected together. The fact that it keeps getting mentioned should be a clue that it doesn’t mean simply being relaxed and moving – there’s something deeper going on here.
It’s referring to the elastic quality of the body that develops from continued practice in qigong or Tai Chi exercise. It’s hard to find because initially there’s nothing to feel, but over time it gently reveals itself through continued practice. And when it does you finally understand what it means to say that when one part moves, all parts move. When the body is connected together a movement in the middle should be felt throughout the body.
The Tai Chi Notebook 
Another excellent topic, Graham. I think this particular phrase is important because it is so easy to misunderstand, and that can be an impediment to skill development.
I appreciate that your post prompted me to reread the Taijiquan Treatise. “One part moves, all parts move” doesn’t mean that every part moves in the same way. In fact, the context in the Taijiquan Treatise seems to point in the opposite direction. It begins with yin and yang, says that movement differentiates, and then gives us the images of standing like a scale and moving like a wheel with the waist as an axle. The Chinese scale is asymmetrically balanced because its parts serve different functions, and the movement of the wheel is unified through rotation even though its different parts are moving in contrasting directions.
So, “one part moves, all parts move” seems to be less about everything doing the same thing and more about nothing moving in isolation, which I think connects nicely with your second quotation about the whole body being threaded together through every joint. Some parts may move visibly, while others stabilize or provide an axis around which movement occurs. Some transmit, and some store or release energy. Yet they all participate in one coordinated event. The parts remain distinct, but they are connected within a continuously changing whole.
That seems to me an important distinction.
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