Xka and Xpa are optionally substituted for the visarga when followed by a hard Guttural or a hard labial.,
Thus वृक्षᳵकरोति or वृक्षः करोति, वृक्षᳵखनति or वृक्षः खनति, वृक्षᳶपचति or वृक्षः पचति, वृक्षᳶफलति or वृक्षः फलति ॥ The क and प in ᳵक and ᳶप are for the sake of pronunciation only. The substitutes are the Jihvamuliya and the Upadhmaniya : two lost sibilants belonging to the class of क and प respectively.
When the rule 8.3.34. does not apply, then this sutra will apply; and will debar that. But 8.3.35. will not be debarred. As वासः क्षौमम्, अद्भिः त्सातम् ॥ There is no vipratishedha between 8.3.35. and 8.3.37, because of the asiddhahood of one with regard to the other. (VIII. 2. I). In fact, every rule in these three chapters stands by itself, and ignores the existence of the subsequent rule. Hence 8.3.35 would not have been debarred by 8.3.37.
To get this, some divide this sutra into two :- (1) कुप्वोः \The visarga is the substitute of visarga before a guttural or labial which is followed by a sibilant.\ (2) ᳵक ᳶपौच The jihvamuliya and upadhmaniya are substitutes of a visarga before a guttural and a labial in every other case.\,