The last of the eight secrets of Ou Yangxun's calligraphy is: (4) How do you understand this? thank you

The ancients practiced the brushwork for beginners, requiring them to bend the pen three times, that is, the left hidden front turns to the left to bend the pen sideways, the two corners are arched upward to form a square, the middle is slightly flat to the right, the tail pen is slightly bent to the right, and the right bottom corner is square, and the pen is gradually lifted to the left and then collected. However, when actually writing Chinese characters, because the drawing is often adjacent to other strokes, it can be exposed to the pen radially, and the end of the drawing can be exposed to the pen and separated from the paper. Beginners can return to the left to collect the pen, so that the bottom of the painting is flat and flawless. If you practice hard for a long time, master the tricks and know how to turn your wrist clockwise to press the top, then it is not difficult to draw a picture. It has the shape of a blade and is smooth and flawless from top to bottom.