Jupiter, being the Devaguru, by nature is a peace-loving planet who signify harmony, universal brotherhood (Vasudaiva Kutumbakam) and wisdom. When such a planet goes to the house of parakrama, he is rendered ineffective under the violent temper of Mars. This is why Jupiter gets debilitated where Mars gets exalted. He can't withstand the violence of Mars.
Jupiter being a karaka for dharma, becomes very uncomfortable when it goes to the house of copulation. Like Sun who is rendered ineffective in the house of pleasures of Bed, Jupiter is also rendered ineffective in the house of sex.
Jupiter is again the prana of the Life force and hence also called the Jiva, where as Mars is the Mrityu karaka. When Jupiter goes to the 3rd house, the house of death, the life force diminishes, more so under the effect of Karaka Mars.
When we look at the ayana of planets we see that the ayana of Rahu is opposite to that of Jupiter. Hence wherever Rahu is strong, Jupiter is rendered weak. In the Kalachakra we see that the Kala of the Dik goveren by Jupiter, is ruled by Jupiter and hence any Jupiterian activity is prohibited in the Rahu Kala.
VENUS IN SIXTH
Jaimini has given a dictum Shasta Shukra Marana Karaka, which says Venus in the 6th becomes Marana Karaka. Why this is so?
Sixth is the house of Service and Venus is the karaka of Pleasures, the two opposites. What shall we expect when the chief karaka for pleasure is asked to serve others! In the natural zodiac, 6th is house of debilitation and the sign Virgo symbolizes a virgin (the purest). Hence at this place Venus becomes incapable to enhance its own Karakatva.
The two karakas for the 6th, Mars (Enemies) and Saturn (Servant). In the Kalachakra this is clear that Mars obstructs the ayana of Venus. Hence none of the Venusian activity shall be performed in the Mars's Kala. Moreover Saturn signify hard work, which is not suited to pleasures and luxury seeking Venus and hence she goes to the Marana Avastha, if placed in 6th.
SATURN IN LAGNA
Saturn is the Karaka (Significator) of destruction of intelligence (Dhi) and opposed to Satya, being opposes to Jupiter and Sun respectively. Lagna stands for the intelligence and health of the native and Saturn is opposed to the karakas of the Lagna, Jupiter and Saturn. Whenever Sat goes to the first house, it can't stand the Karakas and goes into the marana Avastha.
Now in the natural zodiac too Saturn gets debilitated in the first house Aries, where the fiery Sun gets exalted. Here Sun opposes the lethargy and sluggishness of Saturn and wants it to be fast, jovial and forces it to promote the significations of Sun. Now in mythology, Saturn represent Mahakala, the son of Sun from Chaya. Because of treachery of his mother, Sun can't stand him and wherever Sun becomes the strong karaka, Saturn becomes weaker.
Reference: http://new-generation-witch.blogspot.com
