What is the "leak mark" in calligraphy seal cutting?

Leak mark: a metaphor for the artistic effect of painting and calligraphy. The house leaked and the rain ran down the wall. After a while, it will pour down, and it will surely meander down the uneven wall, forming a very frustrating and powerful trace. The vertical painting of calligraphy should pursue such artistic effect. This metaphor was invented by Yan Zhenqing. On one occasion, he asked Huai Su, a cursive master, for his brushwork. Huai Su compared the peaks of summer clouds, birds in the forest, snakes in the grass, and roads to the wall (traces of wall cracks). Yan Zhenqing said, "What about the leak marks?" Huai Su happily shook his hand and said, "You got it."