Bhashya
वाच्यम् ।ततश्च सामर्थ्यावधारणपूर्वकमेव उपास्याकारावधारणमिति अर्थात्प्राक्संबन्धः, पाठश्च अनन्तरमिति—कथं नु संपुटीकरणं स्यादिति प्रजापतेर्हृदयम् ।किंच उपासनान्तरं पठितापि इयमुपनिषत् शक्तिबीजनिर्णयात्मिका तत्र तत्र पदपादसाङ्गोपासनावतरणे ‘यस्मात्त्वमहिम्ना’ ‘यः स्वनहिम्ना’ इति वीप्साशब्देन च अपकृष्टा सती पूर्वं संबन्धं लभते— महोपनिषदपकर्षणवत् ।अतश्च आरम्भणीयेति सिद्धम् ।
In plain words
Here is the answer: you only fix the shape of what is to be meditated on after you have first settled that the mantra is fit for it — so a prior link is already implied, and the recitation follows right on its heels. But then how could the two ever be clasped together as one? That is the 'Heart' of Prajāpati. And notice this: even though this mantra-passage — built entirely to pin down the power-syllable and the seed-syllable — gets recited in between the meditation sections elsewhere, wherever a word or a verse-quarter or a meditation with its limbs is introduced, that same refrain ('because you, by your greatness...') pulls it back into its rightful place, the way the great Upaniṣad-passage is pulled back into its place when it is quoted elsewhere. So it is settled: this is exactly where it belongs.
Close to the Sanskrit
It is to be said [thus]: only after ascertaining the [mantra's] fitness is the form of the object of meditation ascertained — hence there is, by implication, a prior connection, and the recitation follows immediately after. [But] how, indeed, could such a binding-together come about? — [this concerns] the 'Heart' of Prajāpati. Moreover, although this 'Upaniṣad' [mantra-passage] — whose very nature is the determination of the power-syllable and the seed-syllable — is recited in between the several meditation-sections, at each place where a word, a quarter-verse, or a meditation together with its limbs is introduced, it regains, through the refrain-word ('because you, by [your] greatness...', 'who, by his own greatness...'), its prior connection even though drawn away from its place — just as happens with the drawing-out of the Mahopaniṣad. Therefore it is established that it is [rightly] to be taken up here.