Yoga in astrology generally refers to a particular arrangement of Planets which yields significant influence on a Horoscope. According to my teachers one of the best way to learn astrology is to understand the underline basic behind a yoga.
One most common Yoga often called is Gaj-Kesari Yoga.
According to Classics Gaj-Kesari Yoga is formed when –
Jupiter occupies a Kendra (Cardinal House) house without debilitation and malefic influences
Moon is posited in a Kendra from Jupiter
Some classic text only talks about Jupiter being in Kendra from either Ascendant or Moon. While some others only call it just a Kesari Yoga (e.g. Phal-deepika).
However generally it is agreed that Jupiter and Moon in mutual Kendra causes this yoga.
According to classical texts Gaj-Kesari yoga confers great virtues to native and it is described that one with this yoga will be –
“Splendors, wealthy, intelligent endowed with many laudable virtues and will please the king”
“The native will destroy the band of his enemies. He will be a lofty speaker in an assembly and will serve a king. He will live long and famous”
However it is important to understand the underline principle in this Yoga.
This Yoga reinforces that “Kendra” or “Cardianal” house are the most energetic houses of Horoscope. Please check my post on Kendradhipati for this.
Even two of the most benefic and soft planets (Moon and Jupiter) in Kendras, can make a person “destroy his enemies”.
Basically the Kendra positions of Jupiter and Moon not only multiplies the effect of these planet on the native but due to forceful nature of Kendra houses, these characteristics obtain a great strength which manifest very visibly and in the native’s character.
Now as astrology student we should be able to replicate the same behavior with other planets. E.g.
Moon and Venus in Kendra can make the native highly sensual (Venus properties will be actively reinforced)
Malefic Saturn and Mars in Kendra can make the native’s life full of struggle. The native’s success must be followed by perseverance and struggle.
Similarly Sun and Saturn in Kendra (if Saturn is quite strong and maleifc) can make the person see many up and down in life and so on.
Thus every Yoga or combination of planets should be understood in a way to learn the basic principle it is based on.