Source: TW
Separation from mantra-sAdhakas
At some point in the medieval period, the dattAtreyan ascetics separated out of the matrix of the shaiva, vaiShNava and bauddha mantra-sAdhaka-s. They began propounding a new program of yoga, which held rAjayoga to be the highest form of practice. This is made explicit in the dattAtreya’s yoga taught to another atri, sAMkR^iti.
Teachings
It has two sub-teachings: the aShTANgayoga attributed to yAjnavalkya and the haThayoga of kapila. The teaching of dattAtreya has come down to us in multiple texts, primarily the dattAtreya-saMhitA or yogashAstra, and the yogatattvopaniShad:
योगो हि बहुधा ब्रह्मन् तत् सर्वं कथयामि ते ।
मन्त्र-योगो लयश् चैव हठ-योगस् तृतियकः ॥
राजयोगश् चतुर्थः स्याद् योगानाम् उत्तमः स्मृतः ।
Yoga, O brAhmaNa, is indeed of many kinds;
I shall relate all of them to you. mantra-yoga, and likewise laya-yoga, and haTha-yoga as the third. rAja-yoga is the fourth, known as the highest among the yogas.
Attitude to mantra-yoga
This sect saw mantra-yoga – the yoga of their mantramArga predecessors – as the lowest form of yoga:
अङ्गेषु मातृका-न्यास-पूर्वं मन्त्रं जपेत् सुधीः ।
एकेन चापि सिद्धिः स्यान् मन्त्र-योगः स उच्यते ॥मृदुस् तस्याधिकारी स्याद् द्वादशाब्दैस् तु साधनात् ।
प्रायेण लभते ज्ञानं सिद्धिश् चैव+अणिमादिकाः ॥अल्पबुद्धिर् इमं योगं सेवते साधकाधमः ।
मन्त्र-योगो ह्य् अयम् प्रोक्तो योगानाम् अधमः स्मृतः ॥Having first placed the mAtR^ikA-s (Skt letters) upon the limbs, the wise one should repeat the mantra. Even by a single [mantra] one may attain success; this is called mantra-yoga.
It is for practitioners of soft constitution; through practice extending over twelve years, he might attain knowledge, and also the magical powers beginning with aṇimā (minuteness).
A person of low intellect practices this yoga, the lowest among practitioners. This indeed is called mantra-yoga, known as the lowest of the yoga-s.
This is a drastic break from the mantra-mArga.
yogi reaction
The follower of a popular South Indian yoga guru was excited when I pointed this out to him. He said it was clear evidence for an emerging rationality among medieval Indians, who saw the much exalted mantra-practice of yore for what it really was – an ineffectual method for the physically weak. It was to be supplanted by a more vigorous practice focused on physical techniques.
Irrespective of whether there is any truth to this interpretation, it does appear that a divergence from the mantra-mArga emerged among the later nAtha-s (who came out of a clear mantramArga stem) and a strand of the vajrasAdhaka-s among the bauddha-s. Both of these sought siddhi-s through either increasingly physical yoga-s or chemical mercurials. From them it was likely transmitted to the advaita-vedAntin ascetics. This development considerably influenced the lay Hindu outlook towards their religion too.
A third layer to this evolution was probably ramnaam-vaad, taking off in jnaneshwar, naamdev etc’s dismissal of rasayana and hathayoga to full blown sectarian mockeries of gorakh’s post yuganath hathyogic world by bairagis, kabirpanthis.
That could be seen as the latest phase though as you note has deeper roots. Or more generally nAmasaMgIti.
padmAsana + khecharI
The teaching of the padmAsana by dattAtreya suggests that it incorporated the khecharI-mudra within it. Note the specification regarding the tongue touching the rAjadanta. A good part of the yoga tradition only specifies this Asana.
चतुराशीति-लक्षेषु
आसनेषूत्तमं शृणु । आदिनाथेन संप्रोक्तं
पद्मासनम् इहोच्यते ॥उत्तानौ चरणौ कृत्वा
ऊरु-संस्थौ प्रयत्नतः । ऊरु-मध्ये तथोत्तानौ
पाणी कृत्वा ततो दृशौ ॥नासाग्रे विन्यसेद् राज-
दन्तमूलं च जिह्वया । उत्तभ्य चिबुकं वक्षस्य्
आस्थाप्य पवनं शनैः ॥यथा-शक्त्या समाकृष्य
पूर्येद् उदरं शनैः । यथा-शक्त्यैव पश्चात् तु
रेचयेत् पवनं शनैः ॥इदम् पद्मासनम् प्रोक्तं
सर्व-व्याधि-विनाशनम् । दुर्लभं येन केनापि
धीमता लभ्यते भुवि ॥
Among the eighty-four lakh postures, hear of the supreme one. Taught by AdinAtha (usually understood as shiva, as the teacher of physical yoga), it is here called the padmAsana.
Placing the feet upward, firmly upon the thighs with effort, and likewise placing the upturned hands in the middle of the thighs, then fixing the gaze –
one should set it at the tip of the nose, and with the tongue touch the root of the rAjadanta; raising the chin and placing it upon the chest, one should gently restrain the breath.
Drawing it in according to one’s capacity, one should slowly fill the abdomen; then, again, according to one’s capacity, one should slowly release the breath.
This is declared to be padmAsana, the destroyer of all diseases. Rare indeed is it attained on earth by scarcely any, even among the wise.