The idiom force penetrates the back of the paper
Pronunciation lì tòu zhǐ bèi
Definition Li: the power of the pen; penetrating: passing through. The calligraphy is described as strong and powerful, with the strokes almost reaching the back of the paper. It also describes poems with profound ideas and concise words.
Source: Tang Dynasty Yan Zhenqing's "Zhang Changshi's Twelve Yi Brushwork Notes": "His use of the edge is often intended to penetrate the back of the paper. This is the ultimate success."