The most widespread idea is that doing asanas — bending the body into yoga poses — is the practice of hatha yoga. The most well-known text on hatha yoga is Hatha Yoga Pradipika authored in the 15th century. Reading it, one cannot fail to appreciate that classical hatha yoga is not merely practising asanas.
The common meaning of the word hatha is to be stubborn or tenacious — to persist in something with effort. The commentaries on Hatha Yoga Pradipika explain hatha as “with strength.” Here, the strength needed is not only mental, in the form of willpower or persistence, but physical too. A point to remember is that while the word hatha refers to stubbornness, it does not mean blind coercion or compulsion.
It does not mean forceful practice, but practice where a degree of strength is necessary for success. The word yoga in the hatha context generally means “union.” (Unlike as in Yogasutra, where, more logically and usefully, it means “focus.”) Union of what? Of the prana represented by the sound “ha” and the apana represented by the sound “tha.”
However, in truth, these are not two separate functions or energies — they are two aspects of the same. By channelling the flow of prana, the flow of the mind is also channelled and vice versa. The mind becomes focused. Further, the sound “ha” also refers to the sun and the sound “tha” to the moon, or heating and cooling at the body level. The foundation for both of them is agni, the metabolism at the level of the body. What brings them together is vayu or air in the body, that is the movement of the life force or prana.
The prana moves through the channel to the right of the spine centre, called the pingala. The apana moves through the channel to the left of the spine centre, called the ida. The aim og hatha yoga is to bring the prana and apana together, that is, to channel the flow of apana in the central nadi. By sustained effort, both physical and mental, while the person is young and strong, the yogi unites the forces or energies of the prana and apana.
What does uniting Prana and Apana mean?
Generally, when two objects or forces unite, they lose their distinct qualities and identities and take on a composite nature instead. Prana and apana are not two separate forces or entities. They are two aspects of the same force, represented in the breath as inhalation and exhalation. There is only one breath, of which inhale and exhale are two aspects. We can think of it like temperature. For example, temperature is merely a fact of the environment. But when it is greater than our body temperature, we say it is hot, and when it is less than our body temperature, we say it is cool.
Similarly, the classical texts separate the unitary breath into two components — the prana, connected to the sun, and the apana, connected to the moon. This division is representative, not absolute.
When the unitary prana flows in both the right and left channels as prana and apana, the mind is scattered. When the unitary prana, meaning both prana and apana, flow in the centre channel known as sushumna, the mind is centred and focused.
Hatha yoga leads to a deep focus of the mind, and to the different states of meditation or samadhi described in Yogasutra. Thus, the practice of hatha yoga becomes a pathway to Raja yoga, the yoga described by Patanjali in Yogasutra.
Source: https://www.thehindu.com/society/history-and-culture/what-is-hatha-yoga/article25058923.ece