The word "扣人心弦" is formed by sequentially combining the characters 扣 (meaning to grip or fasten), 人 (people), 心 (heart), and 弦 (string), where 心弦 together metaphorically represents "heartstrings". It literally constructs the phrase "to grip people's heartstrings", with 扣 acting upon 人's 心弦 to convey the idea of something deeply affecting or thrilling.