In proṣṭhapada and its synonyms, the first vowel of the second member gets the vṛddhi before a taddhita affix meaning 'born in that time' and having an indicatory ñ , ṇ or k ,
The word ज means 'born', the affixes denoting 'born under that asterism' are meant here. प्रोष्ठपद is the name of an asterism, the time appertaining there to is also called प्रोष्ठपद (the affix अण् of 4.3.3, is elided by IV.2. 4). प्रोष्ठपदासु जातः = प्रोष्ठपादो माणवकः (with अण् 4.3.16). Why do we say 'born in that time'? Observe यदा प्रौष्ठपदोमेघः (= प्रोष्ठपदासु भवः) धरणीमभिवर्षति ॥ The plural number प्रोष्ठपदानां indicates that the synonyms of प्रोष्ठपद such as भद्रपाद are also to be included.,