This is the fourth general rule of writing in cursive hand. To read more about other rules, please visit the learning section of this website, which contains more cursive script tutorials.
Cursive script (草書) ought to flow. Consequently, corners and and straight lines (although not all of them) are often represented by curved lines and loos, various bent lines, arches and so on.
It is not a technique that can be applied freely and randomly to any character and any stroke, as one pleases. There are rules that need to be followed, but those rules are rather difficult to explain in words. They are more of an intuitive nature, which sense is built gradually through studies of classical literature written in other four scripts of Chinese calligraphy. This is why only diligent studies of mainly standard and clerical, but also semi-cursive scripts are essential to understanding and proper execution of cursive script.
View the video (below) to visually see how this rule is applied in writing in cursive script.
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