Lets start by clearifying, that we are not talking of the natural karakas as stated in BPHS...
we are talking here abt the Jaimini karakas . There are seven karakas which are judged in terms of the planet which is highest in degree to that which is lowest in degree, regardless of sign placement.
The karakas are, in descending order of degree,
1. the atma karaka (the indicator of self),
2. the amatya karaka (the indicator of career);
3. the bhatri karaka (the indicator of siblings and father);
4. the matri karaka (the indicator of mother and education);
5. the putri karaka (the indicator of children, intelligence and creativity);
6. the gnati karaka (the indicator of strife, disease, and spiritual sadhana);
7. and the stri or dara karaka, the indicator of marriage (and partnerships in general).

I wud also like to mention here, that most esteemed astrologers exclude Rah and Ket for this purpose, so ur looking at rest of the seven planets.

which planets are Rah and Ket?

Rahu and Ketu are lunar nodes, also known as dragon head and dragon tail....they are very very important part of vedic astrology but are not at all considered in western astrology..

A good question which many experts in the board will share their views.

First What is the meaning of Karaka. If it  can be translated in to english, it can mean the 'doer or that which is responsible for any action or reaction'

You know that 6th 8th and 12th  houses are considered to be evil and destructive houses in vedic astrology.

If an planet is placed in evil houses it normally destroy that planet. I think you would have learnt that too in your lesson.

If you put all the above  logical statement together one can come to a conclusion that the  Karaka loses it power

But I hasten to add, VEdic astrology is not so  simple. It has various permutation and combination of Planets and it prediction should not be made taking into only one aspect or statement in the Dictums.

I wish to elaborate this further

For instance, Assuming Mercury is the Atma Karaka in an horoscope.
Though it would have been placed  in a good house , if it has an evil aspect or situated in a unfriendly or weak house or under the influence of malefic argala,  the good effect of the Atma Karaka may have some dent or for that matter it may not be felt at all. Similarly, the statement a Karaka placed in any of the houses you mentioned loses its strength , may gain strength due to various other factors .

One  will appreciate  the science of Vedic astrology, as they practice  with very many charts and analyse them thoroughly and do research

Hope my attempts to clear your doubt is successful

Lord of Asc is Atma-Karka,and if in the 6th,8th,12th,that means full life fighting with related house,like in 6th saying fighting with daily works,in 8th fight with death related articals,and if in the 12th that means fight with religions and atmonnati.