Since post-Qing Dynasty, about 1911 and the "popularization" of Tai Chi in China the few Tai Chi Grandmasters "simplified" the "forms." This "simplification", circa 1950's was avidly and no doubt promoted as "Beijing 24 movement Form" with appropriate governmental intrigue per the PRC. It was taught to the general public but also to internees in the many Communist "re-education" camps.In simplification, they removed more than the "internal discipline" that you see here: Internal Discipline
In removing what the essence of Tai Chi movement was, they also did away with the need for explanation. The learning was made more difficult in doing this because it became an act of constant memorization of a sequence of movements with no explanation of rationale. In so many words in teaching the "internal discipline." people were told and to this day are told to "just follow along, and you will get it."
Internal Discipline enables you to initiate movements specifically and exactly from the torso above the hips, the internal core of the body (abdomen and back) rather than from the external parts of the body (the limbs) and even the hips in some cases since they are a major joint of the body. It cultivates and mobilizes your internal energy for health benefits and martial arts applications.
Classical Tai Chi or Tai Ji utilizes the philosophy of yin and yang in every aspect of its practice. For every internal movement, a moving part of the body "yang" works against a stationary part "yin" that provides support and power. There then exists what is called a "junction" between yin and yang and it always is located in the torso. When an internal move is performed correctly the practitioner feels sensations of stretching and intense energy flowing across the junction in an otherwise relaxed body.