What is a Yin-Yang Junction and why so important for Tai Chi?

I read many emails in various Tai Chi Forums on a regular basis.  I am quite shocked to see that there are many Tai Chi afficionados who not only do not know what internal discipline is but they do not know what a Yin-Yang junction is.  My teacher Stephen Hwa states what it is in his book "Uncovering the Treasure":  "The most important instruction on Internal Discipline passed down from Wu Chien Chuan to my teacher Young Wabu is that":  "Every movement in Tai Chi Form has to have two complementary parts of the body, a moving part (called Yang) and a stationary part (called Yin).  When the yin-yang junction is located in the torso of the body, it is an internal move.  When it is outside the torso, it is an external move".

I include Youtube links to  2 videos below that were excerpted from a  seminar at Rochester World Tai Chi Day.  In the videos, Master Hwa explains it all by demonstrating and then teaching a group of students.  He uses what is called a "Turning" move with internal discipline to demonstrate.

What is a Yin-Yang Junction?

How to find Yin-Yang Junction, how to do turning, what is internal energy?

Normally, one would expect that it would only be people not trained in Classical Tai Chi who would turn at the hip and carry the torso with it, while the legs have a twisting motion.  However, he recognizes several people who participated in a last years seminar in which he demonstrated this very thing. In fact, I was filming the seminar and I recognized them as well.  In one case, the practitioner is turning his hips so much the legs are not only twisting...they are dancing, with the feet moving all over the place.  You can see this almost immediately, it is so obvious.

You also do not want to move the arms and shoulders as you do this and it can be checked by standing in front of a mirror.  Also, there is a tactile feedback sensation from abdomen and back...but there should be no sensation of feedback from shoulder and arm.  You can read more about this on page 4 and 5 of "Uncovering the Treasure" available at Amazon.