We may get a clue to this by looking at how a sensory event (a citta vithi) starts.
1. A pañcadvāra citta vithi starts when a pañcadvāravajjana citta disturbs the bhavanga state and makes it “come off the static “bhavaṅga state”. That takes three thought moments of AB (atita bhavaṅga), BC (bhavaṅga calana), BU (bhavaṅga upaccheda). Here “atita” (or “atheetha“) means “past,” “calana” (pronounced “chalana“) means “move” or “vibrate,” and “upaccheda” (or “upachchéda“) means “stop” or “break away.”
– That is explained in #4 of the post “Citta Vīthi – Processing of Sense Inputs”
– The same thing happens for a manōdvāra citta vīthi as explained further down in that post.
– By the way, I just revised that post too. I have addressed the issue of a “bhavaṅga citta” there too.
2. The point here is that a new citta vithi based on a new arammana is NOT initiated by a cakkhu vinnana, sota vinnana, ..mano vinnana citta.
– It is initiated by a pañcadvāravajjana citta or a manōdvāra citta
3. I am still not clear about the whole process. But the above is something to think about.