The most likely candidate as a prescription causing one to undertake such study is svādhyāyo’dhyetavyaḥ ‘one should learn the portion of the Veda learn in one’s family’. Veṅkaṭanātha will conclude that this prescription culminates in the learning by heart of the phonemes, leaving aside the grasping of the meaning.
Before that point, however, he analyses the view of those who say that the prescription remains valid until one has studied Mīmāṃsā. These think that each prescription needs an indipendently desirable result (phala). The learning alone cannot be construed as such a result (p. 20 of the 1971 edition), because it is not independently desirable. If one were to construe both the learning of one’s portion of the Veda and the understanding of its meaning as the result (bhāvya), the prescription would end up being meaningless.

Therefore, one should
1. either postulate heaven as the result, according to the Viśvajit rule (according to which one can postulate heaven as result whenever no result is mentioned)
2. or postulate that all results could be achieved, since learning the Veda pleases the deities and the ancestors, who would then grant one all results.

I will come back to why these hypotheses are refuted, but meanwhile, why is it the case that the prescription would become meaningless? Because neither the learning of the sheer Vedic phonems, nor the understanding of the meaning are intrinsically desirable, and each prescription needs a desirable goal.

The Sanskrit passage reads as follows:

svādhyāyārthabodhayos tu bhāvyatve vidhyānarthakyaprasaṅgāt