Hello Astropsychologist.
Midpoints represent points of balance/synthesis between two planets or personal points (eg. Asc/MC) in a chart, but at the same moment they also have an entity/dynamic of their own. The Asc/MC, for example, is not only the point of balance between the projection of the self and the public image; it is also the point of awareness of the identity: the point where you realise how your projection of the self shapes your public image, and how your public image is determined by the way you project yourself.
In theory, all midpoints in a chart – not only the So/Mo – are triggered by transits, progressions, eclipses. Midpoints having a focal point and midpoints without a focal point are nonetheless triggered by all the above.
When studying a chart, I note down at least: the So/Mo, the Asc/MC, the closest midpoint picture, any midpoints having Aries Point at their focal. Also, I check if there is any particularly harsh Midpoint Picture in the Natal Chart, because when it will be triggered the results could be massive. Transits, progressions, eclipses serve as triggers; just the hard aspects though: conjunctions, oppositions, squares. Also, the Solar Return angles conjuncting a midpoint can trigger it and play out.
If you are interested into a midpoints study, I would recommend Midpoints. Identify and Integrate Midpoints into Horoscope Synthesis, by Don McBroom and The Combination of Stellar Influences, by Reinhold Ebertin. Their interpretations are not always on the same wavelength, but I like to look at them in a complementary way.
Vinyasa