Because the whole pig's trotters are not cut, if you answer with a knife, then Shen Wansan will be punished, because in ancient times, you couldn't use a knife in front of the emperor. Shen Wansan had a brainwave, pulled a thin bone from the pig's trotters, and cut the meat with bone, saving his life.
Zhu Yuanzhang thought the meat was delicious and asked what the name of the dish was. Shen Wansan knew he couldn't say trotters, so he turned to say "Wanshan trotters". So Wanshan hoof got its name.