Jerry & Simon, I've been following your exchange and thought I would contribute the following regarding...
"...I'm not very into that sort of 'internal' myself...Like silk-reeling, those aspects of taiji are in traditional Yang style, but the Yangs don't seem to put much emphasis on that or approach it from that direction."
"I imagine that there might have been a change of emphasis when Yang Cheng Fu adapted Ban Hou's form."
From Chen Weiming... one of Cheng Fu's students... who, I believe was also partially responsible for publishing the family manual, prior to the Yang's and who also published some additonal material circa 1930??? (Louis is way more familiar with this than I)... the first from a book titled "Tai Chi Chuan: Its Effects & Practical Applications...
"...attention should be paid to the way you breath. When you breath in, breath through the nose and contract the abdomen; when you breath out, breath through the nose and expand the abdomen. Breath out when you stretch the hand, and breath in when you draw it back. Breath in also when you raise the hand, and breath out when you lower it. Breath in when you separate your hands and breath out when you bring them together. Breath in when you rise up, and breath out when you lower your body.
This breathing is not confined to that of the lungs. It necessitates the utilization of the abdomen. For the stiumulating of the energy of the navel psychic-centre (tan tien) is an essential way to nourish the spirit. It is also closely connected with the shifting of the center of gravity of the body."
....it then continues to give instruction on visualizing and yi with regards to the application (shadow boxing) in the form.
And this also recorded by Chen Weiming... but from the Yang classics (5 Character Secret Transmission??)
"If the chi is dispersed, then it is not stored (accumulated) and is easy to scatter. Let the chi penetrate the spine and the inhalation and exhalation be smooth and unimpeded throughout the entire body. The inhalation closes and gathers, the exhalation opens and discharges. Because the inhalation can naturally raise and also uproot the opponent, the exhalation can naturally sink down and also discharge (fa fang) him. This is by means of the i (mind), not the li (strength) mobilizing the chi (breath)."
Both passages (referring to reverse breathing/microcosmic orbit) suggest that this always has been an important aspect of Yang family Tai Chi... just, seemingly, not taught much anymore.
And, there is so much more information to be found in the classics.
My thoughts. Thanks for listening.