It really boils down to the level of "awareness" an individual has in the process/practice. For someone who is simply learning the physical movements/postures, talking about opening/closing is moot -- they will simply not understand what is being conveyed. Once the individual has learned the physical movements/postures, then inner details can be expounded/described. Is the individual aware of their fascial web? If so, do they have a semblance of control over that?  If so, how well integrated is it within their body? Opening and closing come into play at that point, at a physiological level.    Once this is understood, they can understand and produce the skill required to manifest the effects of said opening/closing. With that, their level of sensitivity will go up. They can now feel qi tangibly, they are aware of how energy moves in spirals. The process of spiraling is also a series of differentials of opening and closing but at a subtler level.    There is opening and closing at the mental level too. The mind when attached to an idea/thought/concept/feeling is closed. When it is not attached to anything, it is open.    It works at the dantien level too -- the dantien opens and closes in a cyclical manner.    That series of interconnected, branching differentials works at many many levels, progressing that way out of emptiness (inner) into emptiness (outer).  My first teacher wouldn't talk about this stuff in a general class setting, because the class would have students at different levels of skill/development. When I started attending private lessons with him, he shared more and more, as time progressed and my skill/understanding improved.