The visarjanīya is substituted for r before a khar consonant or when there is a Pause.,
The word रः is understood. The visarga is the substitute of र् final in a Pada, before surd consonants and sibilants, or at a Pause.
Thus वृक्षश्छादयति 8.3.34, 8.4.40, प्लक्षश्छादयति, वृक्षस्तरति, प्लक्षस्तरति, वृक्षष्ठकारः, वृक्षष्टीकते, प्लक्षष्टीकते, वृक्षश्चिनोति, प्लक्षश्चिनोति ॥ Pause:- वृक्षः; प्लक्षः ॥
Who do we say \before a खर् consonant or at a Pause\? Observe अग्निर्नयति, वायुर्नयति, नार्कुटः (नृ कुढ्यां भवः), नार्पत्यः (नृपतेरपत्यं) ॥ In these two latter, the Vriddhi being considered as Bahiranga, and the र् being the result of such Bahiranga Vriddhi, is asiddha, and is consequently not changed to visarga.
The word पदस्य is understood here, and the genitive should be consrued here as sthana-sashthi, so that for the final र् of a Pada there is visarga, and not for that र् which is not final.,