إِذْ آنَ ارْتِحَالُكَ ... إِذَا زَلَّتْ قَدَمُكَ
When your departure is due ... when your foot slips.
Irtiḥāl is setting out on a journey, applied here to the end of life. Zallat qadamuka evokes a slipping foot; in context it figuratively suggests a disastrous lapse or fall.
The two figures make moral danger concrete through ordinary experience: departure and losing one's footing.