Interestingly, ancient astrological texts, from Parashar to Varahamihir, are silent about it.
This Yoga is said to exist if all planets are between Rahu and Ketu. When that happens in one’s birth–chart, a person experiences many ups and downs throughout life. The suffering increases during transits when the nodes come close to their natal positions. Kaal–Sarp Yoga is considered to be the main reason for a person’s downfall from a high position.
In Indian Puranic tradition, she–demon Singhika had a mighty son whose name was Swarbhanu. He performed severe penance and Lord Brahma was pleased. When he wanted to grant him his wish, Swarbhanu wanted to have the same status in the heavens as planets. The boon was granted.
Subsequently, the Gods and the demons joined hands to churn the ocean in search of nectar. The mountain Mainak was the churning stick and Vasuki, the king of serpents, was the churning cord. A problem arose. Mainak was slipping off to the ocean floor whenever churning was started. To prevent this, Lord Vishnu took the form of a tortoise (His second Incarnation) and Mainak was placed on its back. The churning started and many things started coming out – but those need not attract our attention because that is truly another story. We are interested in the finale – when at last Amrit (Nectar) came up. Amrit (literally translated means ‘No–death’ makes one immortal).
Naturally, Gods and demons were in a mad scramble for it and before anything could happen, Lord Vishnu, in the guise of a beautiful woman (Mohini) took it in her custody and asked Gods and demons to sit in two lines, so that she could distribute Amrit to them.
The Gods knew what was going on and the demons that did not, were so captivated by the enchantress that they readily agreed to do her bidding. The Gods sat in a line on one side and the demons on the other side. Mohini started with the Gods. The demons charmed as they were, failed to see the trick. But Swarbhanu, son of Singhika, with his huge head and body, ending in a serpent like tail, was smart enough to smell a rat. Taking no chances, he broke rank and went to sit with the Gods.
Amrit was served and Swarbhanu was about to take it when Sun and Moon found out that there was an imposter among them. They alerted Vishnu who severed Swarbhanu’s head with his divine discuss (Sudarshan Chakra).
But alas! Nectar had already touched his lips and did its job. The discus had separated the head and the tail and both became indestructible.
Now Lord Brahma’s boon was remembered – and the head and the tail were given the status of planets and placed in the heaven. It is a kind of double and quits or rather the other way round.