idiom · to practice fraud through collusion or to manipulate a situation or documents behind the scenes for illicit gain

他在招標過程中上下其手，最終讓自己的親戚中標。

idiom · to raise and lower one's hand as a conspiratorial hint or secret signal to a collaborator

這兩名證人在法庭上對視並上下其手，顯然有預謀。