Durga Puja - learning Hatha Yoga
Just as Brahma, Shiva and Vishnu represent three manifestations of form, and there are many other forms besides these, Shakti is manifested by numerous forces, powers and energies - but principally as the consort of Brahma, half of Shiva, the Ardhangani of Vishnu, the wives of Ganesh, etc. etc. Durga represents not only the combination of all forces, powers and energies, but the manifestation of their form: she is the combination of Brahma, Shiva and Vishnu - as well as Saraswati, Gauri, and Laxmi. As well as all other beings and non-beings. She is the union of all, good and evil.
Durga is a word that implies a fortress, a defensible position which is invincible. She describes Herself as the bending of Shiva's bow, implying the act itself as well as its intention, and all precedent training. She describes Herself as both father and mother, implying the creative act of not only copulation, but rearing. She describes Herself as consciousness itself. She is known as the Buffalo Slayer, the death of Death, the corruptor of Corruption, the end of Endings. She is known as the All-Woman, the embodiment of femininity.
The reason all beings and non-beings manifested this union Durga was the necessity to defend themselves against destruction and obstruction. It is difficult to imagine a force countering all Dharma, Adharma and Non-Dharma alike, yet this was Durga's enemy. Words are important to conception, so the words "perversely unnatural" (what corrupts a thing's nature) and "betrayal" and "sabotage" (counter-duty, counter-loyalty) may be used, as may "de-loving" (not so much hate, nor unloving, but corruptive of love). "Self-destructive" is also a good term, especially when it is understood that something truly self-destructive will destroy all of its environment with it as well.
Durga's enemy began existence as an Asura, who decided to perpetuate the war between the Devas and the Asuras. Rambha, the King of the Asuras, had fallen in love with a water buffalo and married her. Their daughter was called Mahishi, and their son was called Mahishasura, which means "water buffalo asura."
Mahishasura, before beginning his campaign against the Devas, asked Brahma for a gift of invincibility. But Brahma, finally learning from many previous mistakes of giving similar gifts, instead offered that Mahishasura would only be in danger from being harmed by women. This was good enough for Mahishasura, as he did not believe any woman could withstand his strength. Mahishasura then began a terrible campaign, which threatened to destroy Dharma itself. He knew no restraint, and even hurt himself, knowing that he could not destroy himself (he could only be endangered by a woman). Things got pretty horrific.
Durga, being the ultimate feminine, (and by nature, the union of all beings, also all women), ultimately killed Mahishasura after a long battle, in which he came to regret his gift from Brahma, regret his war against the Devas, and love Durga, regretting his battle against Her. More, in the battle, Durga destroyed the self-destructive tendencies of both the Asuras and Devas, bringing an end to their war.
The story relates numerous shape-shifting by both Durga and Mahishasura, all of which recalls progressions of Asanas. Durga is the bow of shiva, and also the Asana of the bow; Durga is the tree which withstands the buffalo's wrath, and also the Asana of the tree. And so forth. Being all forms, all energies, a Yogi may learn to freely transition between Dharmas, natures and duties, to accomplish their purpose. This is true Hatha Yoga. To a Yogi, there is no masculine or feminine, nor any caste; all is embodied as one. As Durga.