What do you mean, "flip the pen"? What I saw in Guo Yuheng's History of China Literature seems to be the brushwork of words.

Calligraphy step? Fold front, a calligraphy term. A skill of using a pen when changing the direction of strokes. Refers to the pen-receiving gesture, which is different from the pen rotation, that is, when the pen tip changes direction, it turns from the positive side to the negative side, or from the negative side to the positive side. Folding is sharper than stippling and creating postures. Turning the pen is subordinate to folding the peak.