There is the substitution of this stem śīrṣan for śiras when a taddhita-affix beginning with ya follows.,
The word शीर्षन् is understood here. This rule teaches substitution. The original for which this substitution comes is not given in the sutra, we must infer it. The appropriate original is शिरस् ॥ Thus शीर्षण्यो हि मुख्यो भवति, शीर्षण्यः स्वरः ॥ The affix यत् is here added by 4.3.55. The word शीर्षण् retains its original form before this affix यत्, the final अन् not being replaced by anything else 6.4.168. Why do we say when 'a Taddhita-affix follows'? Observe शिर इच्छति = शिरस्यति, here य is not a Taddhita affix, and so there is no substitution.
Vart:- The substitution is optional when meaning 'hair'. As शीर्षण्याः केशाः or शिरस्याः केशाः ॥,