s is the substitute of a visarga when it is followed by a hard consonant (khar) which itself is followed by a sibilant (śar).,
The word खरि is understood here. Thus वृक्षश्छादयति, प्लक्षश्छादयति, वृक्षष्ठकारः, प्लक्षष्ठकारः, वृक्षस्थकारः, प्लक्षस्थकारः, वृक्षश्चिनोति, प्लक्षश्चिनोति, वृक्षष्टीकते, प्लक्षष्टीकते, वृक्षस्तरति, प्लक्षस्तरति ॥
By 8.3.15, the र् was changed to visarga before a hard consonant, or at the end of a Pause. In the present sutra, no special cause being mentioned, the स् change would take place, not only before a hard consonant, but at the Pause also, i.e. in वृक्षः, प्लक्षः also. This however, is not the case, because the word संहितायाम् governs this sutra; so the स् change will be in Sanhita only, and not in Pause, moreover we read the anuvritti of खरि here and so prevent the स् change in Pause.,