This is known as the perfect pose or the pose of accomplishment. It’s used during meditation and pranayama (breathing exercises). To do the pose you place the left heel
against the perineum at the groin area and place the right ankle over the left. This closes off the muladhara (root) chakra preventing prana (life force) from escaping.
Siddha refers to a yogi who has perfected yoga and achieved mastery over mind and body. In doing so they are able to achieve great lightness and consciousness. With that comes siddhis, which are mystical powers such as the strength of an elephant, the ability to change size and shape, and the ability to manifest objects at will.
Durvasa Muni was a renowned ascetic (one who practices spiritual discipline and abstains from all forms of indulgence) who had gained many such mystical powers. He was feared because he had a short temper and was quick to curse people. He could go with or without food for 100 years and later could eat all the food he would have had during those 100 years in a single meal.
Durvasa came to the court of King Ambarish who was a practitioner of yoga and a devotee of Vishnu. Durvasa agreed to be the king’s guest but asked him to wait until he finished bathing in the river. It just so happened that Durvasa came on the day that the king had been fasting as part of a spiritual discipline. It was prescribed that the king should break the fast before sunrise, but it would be disrespectful to eat before his guest returned. When the time had come to break fast, Durvasa still hadn’t come back. The king had a dilemma. He resolved this by drinking a little bit of water, which can be considered food, or not.
When Durvasa arrived he found out the king had drunk some water without feeding him and he was enraged! He created a demon and sent it to ambush the king and kill him.
Although the king had many luxuries, he was not attached to his wealth or his kingdom. Because of his simple-hearted devotion, he was protected by a cosmic weapon, the Chakra of Vishnu. As soon as the demon came near the king, the spinning chakra appeared and killed the demon instead. Then the chakra turned on Durvasa, who fled the scene. Durvasa
wasn’t able to shake it off though. He went to Shiva and Brahma for help but to no avail. Finally he went to Vishnu who told him “There’s nothing I can do. Only if my devotee, Ambarish, is willing to forgive you may the chakra withdraw”. Durvasa went to the king and begged forgiveness. The humble king readily forgave him and called off the spinning wheel. In the end, the mystical powers of Durvasa proved less powerful than the devotional powers of Ambarish.
The true power of Siddhasana is its ability to give the space to meditate on the power and beauty of selfless giving and unconditional love.