"Leak mark" is a metaphor for the vivid lines of works and the vivid aesthetic feeling of calligraphy lines, which has been favored by calligraphers and has continued to this day.
The metaphor of "painting sand with a cone" makes it hidden and easy to paint. When the pen is used, it is often intended to pass through the back of the paper.
Source:
"Tapered sand painting" comes from Yan Zhenqing's "Return to Zhang Changsheng's brushwork" and "leakage mark" comes from the skull's "Sacrifice to a Nephew"