إِنِّي أَرَاكَ تُقَدِّمُ رِجْلًا وَتُؤَخِّرُ أُخْرَى
You put one foot forward and hold the other back.
Used to describe hesitation, the entire forward-and-backward movement is transferred, not just the word “foot.” The borrowed configuration rather than an isolated noun provides the ground for representational metaphor.
If the utterance literally describes someone’s movement, the same words need not be metaphorical at all. The occasion of use is essential; a familiar phrase by itself is not decisive.