Charlie, not that I disagree with you in any way, I am with you, but you are making the mistake of asking him to defend astrology and explain its mechanism, when all he wants to do is determine if there is any coinsitency between readings and reality. That was one of Grey's original disclaimers - that discussion lies elsewhere.
What I have read and otherwise encountered in astrology over the last few decades tells me that astrology does not say that Jupiter does so and so. It only says that so and so will be the trend when Jupiter is over there. In the sense that I can say it will be cold out when Orion is high. I make no claim that Orion is the cause - though perhaps in an astrological world it might be - only that Orion high in the sky means it will tend to be cold. At this latitude anyway.
It is supporters of astrology trying to rationalize the stuff that offer such things as magnetic fields and gravity as functional causes. No astrology text I ever read actually said the planets caused things directly, it was their positions that mattered. No explanation of WHY the positions mattered was offered. It leaves that up to the followers.
No I am not defending astrology, just the form of the argument.