Transits from outer to inner planets I quite believe it's very clear to see... Although I'm suspicious of the modern theory that he inner doesn't influence the outer ones.
They are surely interchangeable, although an inner planet transit to an outer planet is not going to last as long as the outer planet transit to an inner planet.
When, for instance, transit Mars activates an outer planet, whatever that planet's function signifies to the individual could be influenced by a boost of active energy for 4 days or so.
Yet when an outer planet activates Mars, it's the individual's active energy itself that could be either strengthened or weakened over a period of months through years by becoming engaged in, or engulfed by the function the outer planet represents.
'A transit brings it energy to....or highlights .... a natal trait' might be a good way to look at it?
StillOne: I haven't dabbled with solar arc yet
As the name suggests, SOLAR ARC direction motion is purely hypothetical and based upon, and at the same rate of arc as sec. progr. Sun.
This means that the natal planetary aspects remain the same in s.a.d, even though they move continuously forward into other signs and houses. One planet in the aspect can make a s.a.d to another natal planet without the other planet(s) doing so.
S.A.D motions is obviously much faster than secondary progressions, enabling even the slower social and outer planets to make aspects in forthcoming years that are never possible via S.Ps.
There is no logical reason for them to work, yet they do. Personally, I don't work with primary directions but those who do find them just as accurate.
I've told the following stories before because they are both nail on the head verification of chart factors.
My passion for health astrology began the evening my chart was used for interpretation purposes during a lesson. My first and inspiring astrology teacher noted that s.a.d Saturn had conjoined natal Mars in Cancer, thus completing an aspect it had previosly made with natal Moon in Capricorn many years earlier. She was able to correctly state the years when and what instigated a physical complaint I had undergone and.....more importantly.... WHY !
A chart for a client showed an applying sec. progr. Venus conjunct Mercury following a separating s.a.d. Venus conjunct Mercury approx. 18 months or so before. I asked her if she had met anyone during that time and if this initial meeting had progressed (!!) further. Her immediate response was, 'I knew you'd find it
.' She had answered an ad. in the newspaper (Mercury before internet dating!) around that time and had become romantically involved with a widower. He had proposed. She wanted to say yes, yet needed the confirmation she was doing the right thing!
An astrologer on a now defunct site once made the astute comment that a solar arc direction indicates
the direction life is taking. A secondary progression indicates how life is
progressing.
I've never had reason to doubt his words.
As Hannibal(?) of the A-TEAM used to say, 'I love it when a plan comes together.'