For fuller background on haṭha-yoga see this encyclopedia entry by James Mallinson, and on the question why this form of yoga called itself 'force yoga', a essay by Jason Birch.
force, violence, persistence
Stem form: haṭha
Subject form: haṭhaḥ
In ordinary Sanskrit usage the word haṭha occurs most commonly in these adverbial formations:
haṭhāt/by force (literally ‘from force’)
haṭhena/by force (literally ‘by means of force’).
That is how it should probably be understood in the combination haṭha-yoga, ‘yoga by means of force.’ Often in Sanskrit yoga refers not so much to an activity but to a goal or state to be attained.
But exactly why this fierce word was applied here is not entirely obvious. At some point it becomes merely a name and in haṭhayogic texts haṭha is often used, without appending yoga, as the name of the practice.
Traditions hostile to haṭha-yoga (such as that found in the a-manaska-yoga/the yoga of mindlessness) pick up the negative contations of haṭha and complain the practices are hard to master, based in pain and unnecessary. (The view of the a-manaska-yoga is that everything can be achieved by sitting still, fixing the gaze and letting the mind wander: no discipline other than that. A tradition of pure meditation does not see the point of all this other business.)
The origin of the word is also unclear: it neither has any cousins in the other Indo-European languages nor does it have a Dravidian origin. haṭha also happens to be the name in Sanskrit for Pistia Stratiotes, the water lettuce. But presumably haṭha-yoga is not water lettuce yoga.
Water Lettuce, Pistia Stratiotes (Greek: ‘liquid soldier’)
If we include up into the modern period, haṭhayoga/hatha yoga can refer in fact to at least three distinct types:
Firstly, the original version that began to develop from about a thousand years ago: this was not a founding or a launching, but a slow gathering of a suite of elements (to which the name haṭha would only gradually attach). But by the 15th century it had taken on what would be its durable form; and by the 18th century (and the beginnings of British domination) was the predominant version of what ‘yoga’ meant in south Asia.
Among its many features and interests one prominent (and highly visible) element was the expansion of āsana to include a much greater number and variety of poses. Inevitably as they multiplied in number diverse functions and merits were assigned to them. This expansion feeds directly into modern practice (which also likes to assign particular benefits), so there may be a temptation to exaggerate its centrality in the premodern era: āsana was one element in the haṭha tool kit.
By contrast, the (possibly) 4th century yoga-sūtra of Patañjali does not name a single pose, but its main commentary — which may be an ‘auto-commentary’ composed by the same author to explicate the ultra-concise base text of the sutra — lists without description about ten poses (though it does end with Sanskrit’s version of etc). All these seem to be seated poses for meditation.
Secondly, in the modern period ‘hatha yoga’ is frequently used to refer to any and all yoga practice in which postures are an important (if not the primary or sole) element. For instance, David Coulter’s anatomical study of modern yoga practice (one of the best available, if now graphically dated), is entitled The Anatomy of Hatha Yoga. This comes from a separate stream from Iyengar (or Jois; neither are mentioned in its pages), but the difference is minimal.
The original haṭhayoga had opened āsana into a big, variegated category that invited additions. Which duly came: the roster of poses set out in its texts increased steadily across the millennium until the total listed in this or that text was over 200. Then from the late 19th century came a rush of modern additions: the standing poses, sun-salutes. What makes the modern additions distinctive is the variety of sources they were drawn from, including western ones.
Finally, the term ‘hatha’ also gets used to refer to modern styles characterised as “gentle” and “usually for women.” How that came about I don’t know. It may be a claim to be general and not associated with particular modern innovators or developments.
Here is the entry on the word haṭha from p.1287 of Monier-Williams’s Sanskrit-English dictionary, published in 1899 but not yet superceded:
The claims about yoga embedded in this entry are problematic: hard to see what ‘forced meditation’ would be and practices ‘performed with much self-torture, such as standing on one leg, holding up the arms, inhaling smoke with the head inverted etc’ come from the world of tapas ‘ascetic austerities’ rather than haṭha-yoga as such. The haṭha texts do not propose techniques for deliberate stressing like the ‘smoking’ mentioned here.
If anything when they give instructions on how to execute their procedures it is to perform them śanaiḥ śanaiḥ/softly softly. To be sure some practices described are severe or grotesque—such as using a cutting shell, iron tongs and butter to radically lengthen the tongue so it can block the back of the throat—but even here they say do it śanaiḥ śanaiḥ, in this case meaning go slowly and carefully. We may feel this body alteration approaches mutilation, but at any rate the point is not self-harm or self-assault: the difficulty is not the point.
And then among the haṭha poses is śāvāsana, which though it means ‘corpse pose’ (a promising name from an ascetic perspective) is described as the ‘remover of fatigue’. This may not be that far from a modern exercise mentality of work-hard-then-recover. Indeed the ideal yogin is said to have so mastered āsana practice that no fatigue is felt at all.
Alongside its āsanas, that is held bodily configurations that, obviously enough, are visible to observers, haṭha-yoga also deployed internal procedures: this is basically what mudrā means for haṭha-yoga. Some of these are quite mechanical such as the chin or root lock. But even these were deployed in the service of processes that worked inside a body that was not the modern bio-mechanical one. Other interventions had little in the way of hard physical correlate at all: such as trying to stop bindu/semen dripping down from its cranial reservoir, or in the other direction to get kuṇḍalinī ‘the coiled one’ to rise up from the base of the spine and ascend up through the cakras/wheels (which spin when they are activated: before that they are not wheels but knots); until ultimately kuṇḍalinī reaches the summit of the body to release a flood of amṛta/immortality nectar. This (I would be inclined to say) is imaginary, which is not quite the same as non-existent: like much (all?) of human life, they are mental procedures and managed experiences.
It’s quite striking that haṭha-yoga developed (from different sources) elaborate external practices and equally elaborate internal practices. Perhaps in a practice community the external served as a control over the internal: who can tell what stage someone else’s cakras are up to?
Jason Birch points out the occurrence of ‘forceful’ language to describe these internal interventions, both around work done and the ‘response’ inside the body. That is, the language of force is barely used about the external, literal world, but applied with gusto inside the imaginary subtle body. Where at the very least it will do less harm: when you are at work inside a self-created internal landscape you can go for it hammer and tongs. Perhaps that also implies that is where the real work lies.
He notes that the following line (Haṭha-yoga-pradīpikā 3.105) is repeated in four other haṭha yoga texts:
udghātayet kapāṭaṃ tu yathā kuñcikaya hathāt
Just as one opens a door with a key with force
kuṇḍalinyā tathā yogī mokṣa-dvaraṃ prabhedayet
so with kuṇḍalinī the yogin breaks open the door of liberation.
(I don’t know anything about premodern south Asian door locking mechanisms: probably it requires a levering stronger than a modern key, though surely not 'violence'. On the other side of the simile, in the inner world, the force level has evidently gone up.)
This at any rate is an interesting speculation around why haṭha-yoga would be happy to go by the name of ‘force’ — even as it is not, at least in its textual pronouncements, a form of tapas/asceticism.