from Karma and Rebirth in Hindu Astrology, ch. 2
Rahu is the point of obsession in your horoscope, which may be a point of upliftment or downfall. Rahu must perform this act without fail in every horoscope. Every planet in a horoscope is representative of some karma of our past whose fruits, good or bad, we have to have in the present birth. But Rahu, the cause of eclipse, is the cause of some eclipse of something in your life also.
Therefore, see Rahu in every horoscope in the following ways:
1) If Rahu is alone and not aspected by any planet it will give its results according to its own position and the condition of its dispositor (ruler).
2) If Rahu is conjoined with some planets it will eclipse them, and the lordship of that planet or planets falls under the shadow of Rahu.
3) If Rahu is aspected by some planet or planets, notice which they are. Here the broad classification between malefics and benefics in classical astrology can be used effectively.
4) Aspected by malefics, Rahu allows his shadows to fall with violence, as with Mars, if with Saturn, with slow destructive tendencies.
5) Aspected by benefics, Rahu can turn into a benefic.
6) These results can be modified by using the principles of the yogakaraka or sage Parashara. In such cases, Rahu also acts as a yogakaraka.*
reprinted with permission
* Note By Vaughn Paul Manley, M.A.:
In chapter 36 of the Brihat Parashara Hora Shastra, Sage Parashara says:
"If the shadowy planets Rahu and Ketu are situated in an angle or a trine, are in conjunction with or are aspected by the angular lord or the trinal lord, even then they are raja yoga karakas (significator of raja yoga or royal combination)."