Exploring "Shen Fa"; Body Method

Last weekend when I was at a Tai Chi Chuan seminar My instructor took interest in a movement I was doing. The class was practicing "roll back" and "press" from the Yang form. The instructor stood at my left flank and asked that we perform the move at the same time so the class could see how we moved differently. I could not see his exact movement (he was to my left and slightly behind me), so I asked a classmate to explain the difference. My instructor did press in a "long energy" way, with his body pushing the arms as if to project an opponent backward. I was doing press by unfolding my body from the roll-back position in a wave-like fashion, sharply ending in the press like snapping a wet towel or whip. Both were functional in their application, with my method being more "short energy" for penetration rather than projection.
This got me thinking about "Shen Fa" or "body method", and how things I am learning in Bagua and Xingyi are subtly changing my Tai Chi Chuan.
Shen Fa is what makes "internal martial arts" internal. If you imagine a Karate practitioner moving through down blocks and straight punches, you understand how they are powerful but tend to use muscle tension in the limbs and the body and spine remain erect.

Now compare that to this video of Xingyi, most movements clearly using "body method":

Currently on "The Rum Soaked Fist" forum, there is a great three-page discussion on the use of "Shen Fa". In one comment, Kenneth Fish sums up the issue:

"--most of the Xingyi that is public is lacking this - frequently the body, although solid, is like a brick - none of the various wave/rolling/stretching/compressing etc that should be driving and following all movements."

And that is the hallmark of the internal arts; wave/rolling/compressing/stretching.
Take a look at the exercises this practitioner is using to develop "Shen Fa":

One of the next lessons I will have with my Xingyi instructor will be to learn a "Neigong" (internal method) set called "Tuna Si Ba". I have yet to see how the set works, but it takes two hours to begin learning, and 30-40 minutes to practice once learned. I'm starting to believe that my toolbag of fighting forms is looking pretty good, but Neigong internal method may be the most important aspect for me to pursue at my current level of training. I have to say I am fascinated by this. To be clear, "Nei Gong" is internal body work, "Chi Gong" refers more to "breath method" (which I have also studied).
The first part of this next video is of Bagua master Park Bok Nam demonstrating the "Dragon Back" exercise from his Bagua system. This may be the most graphic example of internal body method, with the segmented whipping of the spine in the palm strike. As you can see, the motion is shortened in the stepping strike, but still evident. For the purpose of this article, the first few minutes of this video are the only relevant parts:

Note the coiling/compressing/releasing of this method. This is what really makes the internal arts internal. You see the entire body is involved in the strike, bringing the energy from the root, through the spine, and the power is manifested at the palm.
I love this stuff.
For more information and discussion, check out this thread at "The Rum Soaked Fist".