The affix ṭhak comes in the sense of 'who takes' after a word having pada as its second member, the word being in the Accusative -2nd case in construction.,
Thus पूर्वपदं गृह्णाति = पौर्वपदिकः, so also औत्तरपदिकः ॥ The word उत्तरपद is used in order to prevent the application of the rule when पद is preceded by the affix बहु ॥,