The concept has been talked about among some tai-chi masters, but it was best explained by Yang-style master Wei ShuRen （魏书人） in his "little books" as mentioned in my previous post: authentic tai-chi. Master Wei, learned from Master YongQuan (汪永泉）, has been practicing the old-Yang-style as practiced in Beijing, quite different from the more popular Yang-style as created by Yang Cheng Fu （楊澄甫） later in Shanghai.
The gist of the concept is that the power source of circular movements in tai-chi is conceptualized into three circles: the shoulder circle (activated by the shoulder's ball-and-socket joints, the pelvic circle (activated by the pelvis' ball-and-socket joints), and the waist's circle (activated by Dantian 丹田, lower abdomen - a powerhouse of force generated primarily by the pelvic floor muscles and the diaphragm, which essentially meaning the breathing muscles).
The learning sequence is firstly separate training of each circle then secondly combining the actions of the three circles using the waist circle as the leading entity.
From a practice point of view, or operationally speaking, that means:
1. Use chi to condition (strengthen and loosen) one's shoulder (ball-and-socket) joints; and to make them work together as a whole unit.
2. Use chi to condition (strengthen and loosen) one's pelvic (ball-and-socket) joints; and to make them work together as a whole unit.
3. Use meditation to facilitate the transformation of the action of breathing to generate chi, i.e. synchronize the breathing activity with chi-generation activity.
4. Put the above into action when doing the form.
Of course, the above need some training to perfect; and that lies an interesting aspect of tai-chi!