Rahu and Ketu ?

 When you write on Ketu and Rahu first determine what they are, if a planet or shadow planet, now if you consider them a shadow planet, why are they shadow planets? Next, if they are shadow planets do they cast the shadow or they themselves are shadows? Rahul and Ketu's axis is important considering one and ignoring the other is not the right process. Further, it's said they represent the bhava they are in and the association they have, so in 2nd and 7th one of them is a Marak, having said all let us not forget that they also represent some significations of their own, now the question when do the signification manifest and how? Consider Rahu in the 12th the 12 th house is itself neutral and the lord will give results of the bhava and association or the second sign of the 12th lord, since the 12th lord and Rahu are identical how can one interpret such Rahu? Ketu is Zaid to give positive results with self house mool Trikona and the exalted planet, why? Ketu is like Mars and Rahu is like Saturn, but Saturn is compressive in nature while Rahu expands, Saturn represents austerity and frugality, while Rahu represents indulgence and luxury why the difference?