The idiom "挺身而出" is formed through a vivid metaphorical construction: "挺" means to straighten or stiffen, "身" refers to the body, "而" is a classical conjunctive particle indicating sequence, and "出" means to step forward. Literally, it paints the image of someone straightening their posture and stepping forward, which evolved into a figurative meaning of bravely stepping up to assume responsibility or face a challenge in a critical moment. The phrase thus encapsulates a physical gesture of readiness and moral courage into a single, cohesive concept of voluntary and resolute action.