إن من أسرار البلاغة ولطائفها أن يسكتوا عن ذكر الشيء المستعار
One subtlety of rhetoric is to leave the borrowed thing unnamed.
On this account, the expression introduces an associated feature or consequence instead of stating the source image directly, allowing the hearer to recover it. The mental journey from sign to image is part of the rhetorical force.
Calling a brave man a “lion” directly brings strength to mind. Describing a characteristic connected with a predator can evoke the same source indirectly. Such concealment is not merely stylistic omission; it is a systematic method of figurative indication.