I checked this out, but I couldn't really understand the meaning behind it.
Does it mean good introduction, and good conclusion or something similar?
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It means that the usage of every word is in the person's control. Having an outstanding command of the language, the person knows where to conclude the article and how to put all of the inspirations in. In addition, the person also has a lot of ideas, making the person usually write a lot in a short period of time (also in good quality). As a native speaker, this is my interpretation.
As a native Chinese speaker, I'd say "大开大合" is used to describe something (for instance, a novel/play/movie) that is epic, or grand in scale.
if I want to fly high, I can get to the top of sky, meanwhile if I want to go deep, I can get to the kernel of the earth. This could be called 大开大合