What does it mean to paint sand with a cone, Yin Yinni?

Yan Zhenqing's brushwork effect has the shape of "drawing sand with a cone", Huai Su has the trace of "leaking marks" and Chu Suiliang has the shape of "Yin Yinni", all of which are metaphors of vivid lines of works and vivid aesthetic feeling of calligraphy lines, and are favored by calligraphers and continue to this day. Yan Zhenqing said in the brushwork of returning Zhang Changsheng: "I occasionally draw and write with sharp edges (on the shaping floor). Its strength and danger are obvious and fascinating.