To hold back a skill or some secret knowledge; to keep a trick up one's sleeve.
师傅教徒弟的时候，总是会留一手，不把所有的秘诀都传授出去。
When a master teaches an apprentice, they often keep a trick or two up their sleeve, not passing on all their secret knowledge.
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To hold back a skill or some secret knowledge; to keep a trick up one's sleeve.