For the visarga of namas and puras there is substituted s before a hard guttural or a labial when these words are gati -- puro'vyayam [[1.4.67]] and sākṣātprabhṛtīni ca [[1.4.74]].,
Thus नमस्कर्त्ता, नमस्कर्तुम्, नमस्कर्त्तव्यम्; पुरस्कर्त्ता पुरस्कर्तुम्, पुरस्कर्त्तव्यम् ॥
Why do we say \when they are Gati\? Observe पूःकरोति, पुरौ करोति, पुरः करोति ॥ Here पुरः is a noun, Accusative Plural of पूः ॥
नमस् is Gati by 1.4.74, and पुरस् is Gati by 1.4.67. The anuvritti of अपदादौ ceases.,