Raj Yoga means royal union, it is formed by the association of planets and the houses they rule. Raja Yoga is formed by a planet of a kendra house together with a planet of a triconal house.
The kendra houses (1,4,7,10) are the houses of manifestation and the triconal houses (1,5,9) are the houses of purpose or dharma. The first house serves as both. When planets are combined with both action and purpose great things manifest.
A Yoga karaka is when one single planet forms Raja Yoga by itself. It rules two different houses, one is a triconal and one is a Kendra. The first house can serve as either in the combination.
In both Taurus and Libra ascendants Saturn is the Yoga Karaka.
In Taurus ascendant Saturn rules the 9th (trinal) house, and the 10th (kendra) house.
For Libra ascendants Saturn rules both the 4th (Kendra) and the 5th (trinal).
Other ascendants that have a Yoga Karaka are Capricorn and Aquarius. They both have Venus as the Yoga Karaka.
With Capricorn Ascendant Venus rules the5th and the 10th.
Aquarius ascendant Venus rules the 4th and 9th.
For Cancer and Leo ascendants Mars is the Yoga Karaka.
Cancer ascendant will have Mars rule the 5th and the 10th.
Leo ascendants will have Mars rule the 4th and the 9th. No other ascendants will have a single planet rule both a kendra and a trine. These are the only ascendants to have a Yoga Karaka.
A Yoga Karaka planet will be a planet that brings great benefit to the chart, even though it may be a natural malefic planet.