What is a sand cone in calligraphy?

It means to draw on the sand with an awl, which describes the uniform brushwork of calligraphy without showing its edge.

From Song Dynasty's Continued Book Spectrum and Pen: "Using a pen ... is like drawing sand with a cone ... to hide the front evenly."

Another explanation is that when using a pen, it is slow, and it emphasizes the effect of seeking meaning through doing.