1.
tan/stretch is the ‘verbal root’ out of which are generated (this is how Sanskrit’s own grammatical theory understands it) the array of ‘stretch’ words, both verbs and nouns, for example:
tanomi I stretch
tanoṣi you stretch
tanoti x stretches
titaniṣanti they want to stretch
tānayāmaḥ we make x stretch
tanya needs to be stretched
In Light on Yoga’s roster of poses, the root appears only in the word uttāna. This is found in a handful of the pose names, where Iyengar says it means ‘intense stretch’ — usually the word is an adjective meaning extended or lying on one's back, but in Sanskrit it is very easy for adjectives to function as nouns.
ut + tāna
The ut means ‘up’ but, as in English expressions like mix up, make up, crush up, it’s here not literal upward movement, but rather reaching some high intensity or completed state.
uttānāsana
uttāna+āsana (a+ā = ā)
deep-stretch pose
prasārita pādottānāsana
prasārita+pāda+uttāna+āsana
(a+u = o)
extended leg deep-stretch pose
pārśvottānāsana
pārśva+uttāna+āsana (a+u = o)
sideways deep-stretch pose
pūrvottānāsana
pūrva+uttāna+āsana (a+u = o)
front deep-stretch pose
paścimottānāsana
paścima+uttāna+āsana (a+u = o)
back deep-stretch pose
2.
Modern yoga practice often gets loosely thought of (mostly by people who don’t do it) as ‘stretching’, but it may be helpful to notice the several senses of ‘stretch’ in tan.
stretch = make longer
On the one hand when things are stretched they may become, and be meant to stay, longer.
Notably the early Indo-european languages often gave this a negative implication, where stretched is associated with thin and so weak. For instance, the Latin and Greek equivalent of tan is ten and so in English we have from Latin tenuous and attenuated, behind which lie a metaphor ‘pulled thin.’ The same applies in fact to the English word thin from the Germanic version of the same word base.
As cousin to Latin tenuis/thin, insignificant Sanskrit has the adjective tanu/thin, weak. The only occurrence of the root tan in Patañjali’s Yogasūtra (2.2) is in this sense:
kleśatanūkaraṇārtha
pain+thin-making+for
‘for the sake of attenuating the existential miseries’
stretch = make taut, tense
But there is a different sense in tan, not stretching to make permanently longer, but ‘bringing something into a state of desired tension.’
This is just as much present in the English word stretch: a stretcher in one of its commonest uses is not a device for elongating the patient but one that can carry a person only once it is brought into its state of functional tension. In this sense of the word once something does become longer it may cease to be usable, like a stretched rubber band that is now long and slack.
So from Latin we have tent: a shelter that stands up by being held in tension.
Sanskrit tāna/stretching has a cousin in Greek tonos (source of tone in English) and both are used in a musical context: a particular pitch tension in a string, a musical note produced when just the right tension is applied.
Here stretching means bringing into productive tension.
3.
It is the case that in general āsana practice does tend over time to increase the range of motion a body can operate in. Though it can also be used to reduce it: extreme flexibility, especially weak flexibility, is not necessarily beneficial. All you have to do in that case is to work with force within the range beyond which you choose not to go. The body tends to close off ranges in which it is not asked to operate. In any case, if available range of bodily motion does increase over time, we don’t really experience that on the day.
In the immediate practice itself it may be more useful to think of the āsanas as forms of productive tension, rather than as if they were just a set of elongators. Many poses of course (headstand, tāḍāsana) involve next to no ‘flexibility.’ The tension may be produced by deliberate muscular effort, sometimes just triggered by the demands of have to hold the pose against gravity, or by the inherent resistance of the current body to some move beyond it; but also just in the focus of attention into the pose, which as the word suggests is already in itself a form of tension. The next question, not answered here, would be: if productive tension, productive of what?
4.
A particularly rich and culturally important case of tan/stretch in sense 2 is the Sanskrit term tantra:
tan+tra
stretch + (instrument, means)
This formation is virtually identical to stretcher : stretch + er. The tra is an instrument suffix, a marker that indicates some kind of tool or means. We have a cousin of this marker inside the English words theatre, originally ‘place for viewing’; sceptre , originally leaner, staff for leaning on; and not quite so obviously ladder, also originally ‘leaner.’
Tantra means stretching apparatus, with its most ordinary sense being loom: the frame that holds the threads in tension for producing fabric. From there it has various less concrete meanings, often equivalent to framework or system in English. It can mean for instance governance or government, the controlling system within which activity takes place.
Another of the secondary uses is text or treatise. Text also (literally ‘woven’) is a textile metaphor: just as fabric is created in a linear progression but then when the weaver finishes and steps back it exists as a whole, so a text is created word by word or syllable by syllable and then exists as a whole. It must also be read or heard in the same way, word by word, and then exists as a whole in the understanding and memory. Verbal composition viewed as a kind of weaving is not based on a page/tapestry analogy but was a step was taken by oral cultures without writing where a text was invisible.
The use of the term Tantra and tantric to describe the religious or practice movements which flourished in south Asia in the period 500-1300 CE is more or less modern. It comes from the fact that the texts describing their techniques were very often called tantra (i.e. as part, usually the end, of the text’s name). There are several possible reasons for the word choice: they are texts presenting systems that are also a kind of apparatus in themselves, a form of magic that occurs inside the person.
Characteristic of these tantric practices are internalisation of ritual, that is rituals imagined within the space of the body, and in general manipulation of our internal perception system: often in quite baroque and colourful forms where the body space becomes the universe. (They like other things as well: copulation in cemeteries for instance.) Some of these ways of thinking fed into the haṭha-yogic systems where they are particularly obvious in the elaborately visualised, multi-sensory meditation techniques.
5.
The reason for all the a’s on the left side versus the e’s on the right is that in the language subgroup Sanskrit belongs to (Indo-Iranian), three vowels as found in an earlier stage of the Indo-european languages all converged into a.
a e o > a
ā ē ō > ā
What this means is that when looking for related words in English from Latin or Greek (the Germanic base of English has its own vowel shifts: as in thin above) a Sanskrit a might turn up as any of those three vowels. So for instance:
ship: nāva naval
foot: pāda pedal
stretch: tāna tone