Ive been looking into this a little more, and i've found some information out, like illenek has said there does need to be something going on with the natal synastry first, there needs to be some kind of good connections going on to be the base.
Then you can look at your progressions and see how these work, and this would be progression - progression, applying progressions ( within 2 degrees) would be when things are starting to form, you both might start noticing each other more etc, exact aspects - 2 degrees separating are when the relationship starts fizzling out....unless another progression aspect comes into play. This won't cause an immediate break-up, but a shift will be felt.....this is where the natal synastry comes in, as when the progressions seperate, they need their natal synastry's to be their safety net, to save things and keep them together.
Your only looking at hard progression aspects to bring an event so conjunctions, oppositions, and a trine has been mentioned (?)
Transiting planets can work as a trigger to the natal or progressed aspects.
For example: i have with someone pr venus sextile pr venus, my Pr venus conjuncts his n juno and his P moon is 5 degrees away from hitting my Pr venus/juno.
Now for anything to happen here, we'd need a transiting planet to be hitting this venus/venus sextile to trigger it off, without a trigger it would lay dormant, his P moon is approaching yes and will hit in 5 months time, but im not sure if this is strong enough.