如臂使指 as if by one's own arm
Explanation
比喻运用自如，得心应手，像指挥自己的胳膊一样方便。形容对人或对事物控制自如。
This idiom is used to describe the ability to control something with ease and precision, as if it were one's own arm.
Origin Story
话说三国时期，诸葛亮在七星坛上摆下八卦阵，抵抗司马懿大军。司马懿大军来势汹汹，眼看就要冲破诸葛亮的防线。诸葛亮沉着冷静，指挥若定，他一边运筹帷幄，一边亲自下令布阵。只见他指挥若定，宛如臂使指，轻轻松松地就将司马懿大军引入八卦阵中，而后以少胜多，大败司马懿，取得了战争的胜利。
During the Three Kingdoms period, Zhuge Liang set up the Bagua Formation on the Seven Stars Altar to resist Sima Yi's army. Sima Yi's army came menacingly, and seemed about to break through Zhuge Liang's defenses. Zhuge Liang remained calm and composed, commanding with poise. He planned the strategy, while personally issuing orders to deploy the formation. As he commanded with composure, he directed the army as if his arms were extensions of his will. He effortlessly led Sima Yi's army into the Bagua Formation, and subsequently, by using fewer troops, decisively defeated Sima Yi and won the battle.
Usage
多用于军事指挥、领导管理等方面，形容对人或对事物的掌控能力很强。
Often used in military command and leadership management, to describe a strong control over people or things.
Examples
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他指挥若定，调度有方，真是如臂使指。
ta zhihui ruoding, diaodu youfang, zhen shi ru bi shi zhi
He commanded with composure and precision, as if his arm was an extension of his will.
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老将经验丰富，指挥若定，如臂使指
laojiang jingyan fengfu, zhihui ruoding, ru bi shi zhi
The old general, experienced and calm, directed the troops with the ease of controlling his own arm