As per Vedic Astrology, Rahu-Ketu is seen as the shadow planets in astrology. They are considered cruel, sinful, and separatist planets, known for giving many negative life results. These two planets do not have any physical form. Rahu is exalted in Taurus and Gemini, while Ketu is exalted in Scorpio and Sagittarius. The nature of Rahu is similar to that of Saturn, and the nature of Ketu is similar to that of Mars. Rahu and Ketu always move in retrograde motion, and together these two planets have a special effect on the life of a human being. If Rahu and Ketu are in a favorable and strong position in your horoscope, they can provide many benefits. If Rahu and Ketu are in an inauspicious position in your horoscope, then these two planets together can put you in great trouble.
A general belief about these planets is that they give only inauspicious effects. But the truth is that if Rahu-Ketu is in a promising position in the horoscope, then along with wealth, the person gets a high position and prestige in society. When Rahu and Ketu are in a strong position in the horoscope, then they provide wealth, high position, respect, great success, etc., to the person. When Ketu is strong, a person becomes spiritual and devoted to God. Rahu and Ketu are considered strong in the 03rd and 11th houses of the horoscope. Rahu-Ketu situated in these houses gives courage, might, good fighting ability, and fearlessness to the person. A person whose Rahu-Ketu is in a strong position in their horoscope faces fewer problems in their life. The financial condition of such people is also strong. If Rahu and Ketu are situated in the third or eleventh house, and they are under the influence of auspicious planets, then no obstacle can stop such a person for a long time. They will be able to complete every task easily that they perform in their life.
Now let us discuss Rahu in the sixth house and Ketu in the twelfth house. The sixth and twelfth houses are bad houses except for a few things; hence this house is counted in the trik (6, 8, 12) bhava. One concept of astrology is that if a malefic planet is situated in a bad house, it destroys the inauspiciousness of that house and gives auspicious results. It is just like in mathematics that two negatives combine to become positive; in the same way, it is believed that when a malefic planet is situated in an inauspicious house, it destroys the inauspiciousness of that house and gives auspicious results. The sixth house is related to disease, debt, enemy, dispute, injury, litigation, etc., and the twelfth house is related to loss, expenditure, secret enemy, jail, hospitalization, etc.
When Rahu and Ketu sit in these houses, then according to the above concept, the inauspicious significations of these houses are destroyed. Therefore, Rahu in the sixth house and Ketu in the twelfth house are considered auspicious. But this is not always true in experience. A malefic planet sitting in an inauspicious house can cause double trouble to a person by inculcating the factors of those inauspicious houses, because one is naturally malefic, due to sitting in inauspicious houses from above, it can take up these bad qualities and can give double the inauspicious results. Your second question is that despite this position of Rahu and Ketu, why a person gets success after hard efforts? The answer is that from the sixth house, Rahu aspects the tenth house (work area) and the second house (the house of wealth and economic status), and Ketu in the twelfth house aspects the fourth house (place of happiness). Due to the aspect of Rahu at the 10th house, obstacles in work are there, and the aspect at the 2nd house gives struggle in earning money and status. It is due to the aspect of Ketu at the fourth house a person does not get happiness easily.