In the Rigveda (6-9-1), the terms kṛṣṇa and arjuna are found in a slightly different context than we all see in the Mahābhārata.
कृ॒ष्णमह॒रर्जु॑नं च॒ वि व॑र्तेते॒ रज॑सी वे॒द्याभि॑:।
जाय॑मानो॒ न राजावा॑तिर॒ज्ज्योति॑षा॒ग्निस्तमां॑सि॥ १॥
ahaśca kṛṣṇamahararjunaṃ ca vi vartete rajasī vedyābhiḥ
vaiśvānaro jāyamāno na rājāvātirajjyotiṣāgnistamāṃsi ॥1॥
The two beings (rajasī) rotate alternately (vi vartete), by means of these forces the dark period of the day (night) (ahaḥ kṛṣṇam) and the light period of the day (day) (ahaḥ arjunam) must be known (vedyābhiḥ). Fire (agniḥ), being born (jāyamānaḥ) within each person (vaiśvānaraḥ), like a king (na rājā), coming into being and overcoming (ava atirat) darkness (tamāṃsi) by light (jyotiṣā).
The text of the Rigveda does not directly say what kṛṣṇa (is dark day) and arjuna (is bright day), but I found clarification in Nirukta (2-21) by Yāska, he explains that these are the day and night – ‘ahaḥ ca kṛṣṇaṃ rātriḥ śuklaṃ ca ahaḥ arjunam’ (black and white days are also day and night). In this hymn of Vaiśvānara it is said that Vaiśvānara is beyond sleep and wakefulness, also Śankarācārya in his comments to this verse connects it with a state of consciousness, which is beyond sleep and wakefulness. We can also recall the famous metaphorical statement of the Gītā that when ordinary people sleep, the yogi is awake, and vice versa, e.g. the yogi focuses on dimension that is beyond normal human states. For the yogi, “the battle of Kṛṣṇa and Arjuna” is a process of victory over constantly changing impermanence during sleep and wakefulness. On the other hand, day and night are also forms of the Deities in the Vedas. For example, by falling asleep we give ourselves into the hands of Goddess Ratri, it returns us to our being and thereby restores the vital forces that are lost during the day. Sleep and awakening are a kind of “micro-rebirth,” in tantric yoga there are enough practices associated with sleep or dreams.
On higher levels of initiation and practice of tantrism, such as pūrṇābhiṣeka, medhā-sāmrājya and practices of upper āmnāya, there are methods of contemplation of Ardhanārīśvara. Also, such ritual as pañca-tattva is not usually practiced before śāktābhiṣeka dīkṣā, and this is especially true for higher initiations and āmnāyas. Such titles as left-handed or right-handed tantra have both symbolic and quite practical values, for instance, pātra with wine is taken by the left hand when we offer it to the ”inner fire” into the mouth of Kuṇḍalinī. The left part of the body is Śakti and the right is Śiva. Some tantrikas say that kaula sādhana begins where such dualistic methods as Patañjali yoga are ended. But the same could be said about nāthas and kaulas: nātha sādhana begins where kaula sādhana ends. It has always been like that in India: the more recent endevour to continue something is more substantial than the previous one. That is why Gorakṣanātha is more honoured today than even Matsyendranātha, but this, of course, does not belittle the benefit and the authority of the latter. In the same way, for example, Vedanta – the completion of the Vedas, is not considered as something below the Vedas, but rather as its essence by either Vedantins or many other Hundu. Or, similarly, we couldn’t say that Vajrayana Buddhism is a simplification of Theravada, despite the fact that it had been developed later. Or, for instance, the fact that some cults of early female deities, like sapta and aṣṭa mātrikās have been transformed into such sophisticated cults as Trika, Kubjikā, Śrīvidyā and others, doesn’t make the latter less developed or less authoritative. And the most essential way was always the most secret, with a very careful selection of applicants for that kind of dedication and practice. The same could be said about nāthas. The phenomenon of svara-yoga is of tantric origin, but its basis is still yogic. In speaking of essence, by which we usually mean something that is closer to us as subjects, and also implying the involvement of the subjects in different degrees of the external process. That is why the practices with the body, breathing, with tracking how the breath is associated with the sun and the moon, how these two are related to the elements, tithis, grahas, nakṣatras and other aspects of both micro- and macrocosmos, are very great and subtle processes. But the most important thing is that they are all tied to the essence of it all, namely the yoga of the Sun and the Moon. In many books of both the medieval gurus of the Nāth Sampradāya and the present authors, we can very often find a description of the importance of svara-yoga and, of course, the practices of it. That can be called the basis of nātha-yoga, as its symbol is the Sun and the Moon, i.e. Śiva-Śakti saṃyukta.
Modern popularisation of yoga and competition in this field gave rise to the invention of the multiple techniques, which affects their quality. But, I believe that in the traditional approach the greater the transition from the number of techniques to their quality – the greater the authenticity of yoga.