Well, ok, the focus of Chen's book is not specifically chan ssu jin. You're right; there's a lot of other stuff. However,
The idea of neijin is brought up repeated in the book and if we're discussing peng jin as a core strength, neijin is very similar, if not synonymous.
This is why I stick with discussing the Yang classics. I don't study Chen or Chen theory. The Classics don't call pengjin a core strength or describe is as such. That's a modern thing. Neijin (or internal jin) is the same "core strength" for peng, lu, an, or ji. Hey, there are people who can explain it as something different; but, I can't and don't.
Secondly, the quote you took from Chen's "Illustrated" describes a technique, not an energy or jin.
At end of the section entitled Explanation of Taijiquan’s mechanism of development there is a poem
Verse 1
To conduct such techniques as Ward-off (Peng),
Roll-back (Lu), Press Forward ( Ji) and Push Down (Na),
You must indeed take the Truth to heart.
In order to properly perform such skills as Attracting (Yin),
Advancing ( Jin), Dropping (Luo) and Avoiding (Kong),
You must progress gradually, step by step.
The whole body follows your opponent –
Your closeness creates his difficulties, through which
You can then manipulate the momentum of a thousand pounds
With a force of four ounces!
He calls it a technique, and that's never the same as a jin. If I were a Chen practitioner, I would probably want to read Xin's book more thoroughly. But, from what I've read, Charles's suggestion to avoid it is probably valid. Well, for one thing, the book was finally translated in 2007. So, as I said, few people knew the contents; and the esoteric parts --that you'll see if you read-- seem far beyond the interest of a casual practitioner. For example, find the text on Googlebooks, and look at the table of contents. I congratulate the practitioner who understands and can transmit the details of the He River or Yellow River charts. I'm not sure how many Chen practitioners know these or know the book in general.
The Yang "Songs" were and can be used as mnemonic devices for students. They're things that were said and written a century and more ago and are (or should be, imo) at the core of the "transmission." They serve a completely different function in the Yang lines as they do in the Chens. That is not a criticism, and not even a disadvantage in terms of practice. Plenty of people know the words. It's easy to say "lead him into emptiness, then use four ounces"... etc. There may even be lots of ways to accomplish it, which will ultimately come down to interpretation, and maybe even translation.
Btw, the main reason I came back to this thread was this part:
To conduct such techniques as Ward-off (Peng),
Roll-back (Lu), Press Forward ( Ji) and Push Down (Na),
I don't do Chen style, but I think the translator made a mistake with "Push Down." It should be "An", not "Na" (grasp, hold). In another oral tradition, there's a saying "Hua, na, da, fa," and I know how that's interpreted. But, I don't speak Chinese. So, maybe "Na" is another way to say push down. Ime, in the Yang/Wu tradition, that's not na. Ymmv.
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