Jupiter (Guru) is the planet of divine grace, wisdom, and the voice of the heavens. The 9th house is the house of Dharma, and the 12th house is the house of Moksha and the spiritual realm. Both are natural houses of Jupiter.
When Jupiter sits in either of these houses, the person is granted a terrifying spiritual power: Vak Siddhi (the power of speech).
Whatever this person says from the bottom of their heart will manifest into reality. If they bless someone ("dua"), that person will flourish. But if someone hurts them deeply and they issue a curse ("shrap"), the universe will execute that curse without mercy.
The VAYOM Warning: If you have this placement, you must control your anger. Never curse anyone, even in a fit of rage, because you will destroy their life and carry the karmic burden.