Sutra:
75. inimEl praNavaththai vivarikkiRathu.

Meaning:
The parts other than the praNavam in the Thirumanthram explain the praNavam.

Sri PBA Swami's Sarartha Deepikai:
From here on, the nama: part of Thirumathram which is known as mantra sEsham and the nArAyanAya part have to be explained.

First it is said that the mantra sEsham is an explanation for the praNavam. Breaking a word or phrase into multiple parts and showing the meanings of each part is the usual method of explaining something. So, why would it be said that the mantra sEsham explains praNavam, when it does not break the praNavam part by part and show their meanings?

Even if it does not take the parts of the source and explain them, as it speaks the essential meanings of the source and expands on it, the mantra sEsham can be said to explain the praNavam. That is, the collected meanings of the praNavam that were shown before are explained in great detail by the remaining two parts of the Thirumanthram.