Hamsa is one of the five Pancha Mahapurusha yogas, the great-person combinations of jyotisha, and it is the yoga of Jupiter, the great benefic. It forms when Jupiter occupies its own sign, Sagittarius or Pisces, or its sign of exaltation, Cancer, while standing in a kendra, an angular house from the ascendant. Its name means the swan. The tradition reads Hamsa as conferring the Jupiterian virtues in strength: wisdom, righteousness, learning, a benevolent and respected character, and spiritual inclination, giving a person of the wise and virtuous type. These are the interpretive conventions of the tradition; the yoga is a defined placement of Jupiter in dignity and angularity, and among the five it is often regarded as the most auspicious when the planet is strong and unafflicted.
Jupiter's exaltation in Cancer sits opposite its own sign of Capricorn's fellow earth-triplicity concerns, and commentators have long linked Hamsa's swan imagery to the hamsa of Vedantic literature, the discriminating bird said to be able to separate milk from water, an image of the discernment the yoga is supposed to confer rather than a technical astrological point. On the question of source and reckoning it follows the pattern of the others: Brihat Parashara Hora Shastra gives the condition from the ascendant only, while Mantreswara in the Phaladeepika allows it from the Moon as well, with the two together, ascendant and Moon, said to raise the effect toward sovereignty and exceptional fortune (samrajya). Later compilers such as the Saravali elaborate the physical bearing and temperament of a native said to carry Hamsa strongly, beyond the character summary given above.
Sources Brihat Parashara Hora ShastraSage Parashara (attributed) with SaravaliKalyanavarma