Maybe it’s something to do with the movements: the Cat and then the Cow, the twist to the left and then to the right, the reaching up, and then bending to the ground, the constant training of the body to move one way, and then to move in the opposite way. Hatha: sun, moon opposites, dark and light, yin and yang. This must be key in the way yoga shapes the mind and heart, in the way it helps one to understand that every movement has a counter movement, that every action has an opposing action, that the happy parts of life will be met by the sad, and the sad, in turn will be met by the happy.