The idiom 势成骑虎 is constructed from four characters: 势 (situation), 成 (to become), 骑 (to ride), and 虎 (tiger). It literally sequences these elements to describe a scenario where a situation has developed into one akin to riding a tiger, a metaphor derived from historical anecdotes where dismounting is perilous, thus conveying a state of being trapped in a difficult predicament with no easy retreat.