The deepening steps are the "secrets" of Tai Chi Chuan which enable us to understand the how and why of the movements.
It is pointless to practice Tai Chi Chuan without the deepening steps: even after years of practice there will bei no chi-development.
The six Deepening Steps
The Yin-Yang Form
Yin and Yang correlate with the breathing. This is the first and most important structuring of the form: when is there breathing in, when has one to breathe out.
All Tai Chi movements follow the principle of the spiral. The first spirals one learns are those of the arms, this is called the Chi-Form, because with this deepening step the development of Chi becomes evident to the student.
All movements in Tai Chi Chuan are lead by the "Center", the lower dan tien, which also is the mass center of the human body. In this deepening step the focus is on the correct rotation of this center.
The Leg-Spiral opens the energy-channels in the legs. Thus, the energy can flow freely from the ground through the feet and legs into the hands.
With the Neck-Spiral the upper dan tien is included into the Tai Chi movements. It is also specifically about the energetic connection between the head and the hands.
The Internal Breath is a way of breathing specific to true Tai Chi Chuan. Only with the proper breathing can Chi be developed and used to its full potential. The Tai Chi Chuan form, the Standing Meditation, the Taoist Health Exercises all focus on the development of this "Internal Breath".
This is the final and most advanced of the Deepening Steps which can not be realized without the proper training and preparation.