Definition and Description of Anapha yog from classical text Brihat Jataka by Varahamihira.
Definition
If the 12th house from Moon is occupied by planets other than the Sun, it is called Anapha Yog.
Description
"One born in Anapha yog rules over others, is devoid of disease, is of good character, with a good name, enjoys comforts of life, dresses well and is free from worries."
Varahamihira differentiates this yogas based on the planets involved in the last verses dedicated to Anapha, Sunapha and Dhurudhara Yog in his work Brihat Jataka.
Mars"If the yog is brought about by Mars, the person is full of ambitions, bravery and wealth."
Mercury"If by Mercury, he is clever, with good powers of speech and an expert in arts."
Jupiter"If by Jupiter he enjoys wealth and comforts, is religious and has the favor of king."
Venus"If by Venus he is given to sex, is very rich and lives in luxury."
Saturn"If the yog is created by Saturn, he enjoys other's wealth, house, clothes, conveyances, engages in many activities and is the leader of a group of people."
Explanation
Moon is a very important component of a Birth chart along with Lagna / Lagna Lord. Moon represents almost every nurturing factor of life and this is the reason why Moon is also Karaka for mother, as she being the greatest of all nurturing persons in someones life.
And same is the reason why so many lunar yogas are also associated with wealth and status like Chandra Adhi Yog, Gaja Kesari Yog, Chandra Mangala Yog, Mukuta Yog etc. Ancient Jyotisha classics comprises of considerable number and wide variety of lunar yogas is an indications towards importance of Moon in Jyotisha (Vedic astrology). Jyotisha emphasise the interpretation of a Birth chart from Chandra Lagna, or the position of Moon as Lagna, in addition to the Birth Lagna (Ascendent) gives another clue on importance given to Moon.
Moon by nature is a very impressionable planet (having tendency to be easily influenced by any other planet). She has reflective, receptive, impressionable, feminine qualities.
Moon likes the company or support of other planets, in the same way our mind or manas (in sanskrit) clings to different types of securities. To be noted that Moon is also the Karaka for mind. So by the virtue her nature Moon is acted upon by other planets or we can say she absorbs the qualities and energy of planets within her close proximity, like planets which are in 2nd and 12th houses from her.
Rahu Ketu are Chaya Grahas or shadow planets, they themselves act accordingly of their association and position. It is not mentioned specifically in classics, but Moon’s Node Rahu and Ketu are not included in the formation of these yogas.
In formation of Anapha Yog Moon absorbs the qualities of planet in 12th house from her. The variations given in Brihat Jataka stated above, clearly signifies this.