The role of suṣumṇā in the mantra realisation

Looking through the Bengali text, the Bṛhattantrasāra, written by Guru Kṛṣṇānandāgamavāgīśa, I found an interesting point regarding the mantra practice, where the Gautamīya-tantra is quoted. We know that there is a classical practice of puraścaraṇa in the form of japa, homatarpaṇamārjana and brāhmaṇa bhojana. However, it speaks of a certain “exception”, which, in my opinion, is a transitional moment of sādhanā, “joining” in itself the goals of vāmācāra and yoga of the Nāthas (vajrolī). I decided to share this piece of text with you, maybe it will give someone an additional understanding of the tantra and Nātha yoga practice.

पशुभावे स्थिता मन्त्राः प्रोक्ता वर्णास्तु  केवलाः 
सौषुम्ने  ध्वन्युच्चरिता  प्रभुत्वं प्राप्नुवन्ति ते
मन्त्राक्षराणि चिच्छक्तौ प्रोतानि परिभावयेत्
तामेव परमव्योम्नि परमामृतबृंहिते
दर्शयत्यात्मसद्भावं पूजाहोमादिभिर्व्विनेति
मूलमन्त्रं प्राणबुद्ध्या सुषुम्नामूलदेशके
मन्त्रार्थं तस्य चैतन्यं जीवं ध्यात्वा पुनः पुनः

In the Gautamīya Tantra:

The mantras practiced in the paśu bhava are recited only at the letter level. The Suṣumṇā-related, practiced sounds are overflowing with power. During jāpa, the mantrāksharas should be fully connected to the power of consciousness. That (mantra) in the highest space is nourished by the highest bliss. Pūjā, homa and etc. are not required for this type of practice, practice the main mantra* through the power of consciousness in the root area (the base) of suṣumṇā (genitals). Practice meditation over and over again with that mantra essence, the consciousness of a living being.

Mūlamantra (the root mantra) is the main mantra of a Deity. For example, Śiva’s mantra is oṃ namaḥ śivāya, Gaṇeśa’s mantra is oṃ gaṃ gaṇapataye namaḥ, etc.

Whether foreigners can follow the Vedic tradition or not?

As to whether a foreigner can follow something Indian or not. To clarify this, I will quote from the Yajurveda. I specifically take the Veda, since there is an opinion that one can receive initiation only in Tantrism, the Bhakti or the Nātha traditions, but not in the Vedic one, etc. Here is a śloka from the Śukla Yajurveda (Vājasaneyī Saṃhitā), from which follows that there is no absolute radicalism towards foreigners even in Vedism:

यथे॒मां वाचं॑ कल्या॒णीमा॒वदा॑नि॒ जने॑भ्यः।
ब्र॑ह्मराज॒न्या॑भ्याँ शू॒द्राय॒ चार्या॑य च॒ स्वाय॒ चार॑णाय। (यजु ० २६। २)

I have transmitted the auspicious words of the Vedas for people like brāhmaṇas, for kṣatriyas, vaiśyas, śūdras and for araṇāyas (foreigners).