Daily Chinese Idiom Guessing Game
A person or thing that holds great influence, where a slight move can affect the whole situation.
只要脚移动一下，就会影响两边的轻重。指处于重要地位，一举一动都足以影响全局。
《后汉书·窦融传》方蜀汉相攻，权在将军，举足左右，便有轻重。”
His opinion carries such weight in the committee that his support is crucial for any decision.