In his essay Secrets of Writing (作文秘诀), Lu Xun wrote: “A boxing master holds back a move when he teaches his pupils. He fears that once they have learned everything, they will beat him to death and take his place.”
Doctors have secret prescriptions, cooks secret methods and pastry shops secret recipes. “To protect their family’s livelihood, it is said, these are taught only to the sons’ wives and not to the daughters, so that they do not pass to other families.”
The phrase is 留一手, to keep back one hand. The Handian dictionary defines it as keeping some skills back and not bringing them out. A popular tale tells how the cat taught the tiger everything except how to climb trees. When the tiger tried to eat its teacher, the cat escaped up a tree.