What is the hollow phenomenon of strokes in calligraphy?

Deviation, overvoltage, vain weakness, slow efforts, etc. Will cause the strokes to be hollow. The result of hollow strokes is pale and powerless. There is no content, three-dimensional sense, life, emotion and expressiveness in the strokes. In short, there is nothing to want. If you don't want to be empty, you should practice controlling the strength of the pen for a long time so that the strokes can stick. What is a penetrating paper back? Is to make the strokes powerful. This is just a premise. As for what you want to express, it depends on your accomplishment.