लक्ष्यच्युतं चेद्यदि चित्तमीष -द्बहिर्मुखं सन्निपतेत्ततस्ततः ।प्रमादतः प्रच्युतकेलिकन्दुकःसोपानपङ्क्तौ पतितो यथा तथा ॥ ३२६ ॥
In plain words
Let the mind slip even a little from its true aim, turning outward, and it keeps tumbling further and further away - the way a ball, dropped carelessly at the top of a staircase, bounces down step after step, gathering speed as it goes.
Close to the Sanskrit
If the mind, having slipped from its mark, turns even slightly outward, it falls further and further down - just as a play-ball, dropped through carelessness, falls further and further down a flight of steps.