This question came up in the Online 10-day Retreat Nov 3, 2020, guided by Delson Armstrong. Day 7 was part of a daily 30-minute discussion on the suttas, the Dhamma talk and reflections.
Slightly edited to improve readability
Yes, they are primarily accessed through formations. Formations are the carriers of the kammic impulses. If you go to Digha Nikaya sutta 14, the Mahapadana sutta, the past Buddha Vipassi; when he was reflecting on the links of dependent origination, he went as far as consciousness, because he was looking into just this life. But once you get into formations, you start to access the formations that arose in this present life, due to past kamma in previous lifetimes. The way to access different past lives is to use the fourth jhana as the base, your foundation, because it has very strong equanimity. You need that strong equanimity as you get to experience the previous lives, because there might be great amounts of joy in certain lives, there might be great amounts of sadness in other lives. You need to maintain that equanimity.
The way it’s done is to start to remember things that happened in this life, starting from yesterday and then going back. It’s like a fast rewind which starts to show things, up until the point of one’s birth in this lifetime. And then beyond, which will be the previous life where the body dissolved – if there was mentality-materiality. So, the previous point being the death of the previous life, and so on and so forth.
That’s one way of doing it, using the fourth jhana and then going on and looking at those past lives in that way.
There’s also the ability to see the arising and passing away of beings, according to their kamma. This is the Threefold Knowledge. The first knowledge is seeing the different lives across eons, and one can experience that at a different scale of time. What might seem like maybe an hour, or two hours, or maybe an entire night of having gone through different lives, in that time span you have gone through possibly eons and eons of different lives. Even up to seeing the destruction of the universe, the contraction and then the expansion and so on.
The second knowledge is seeing the arising and passing of beings, depending upon their kamma. You start to see the different realms at that point, you start to see how a certain action created this certain formation in that being. When you see that this action created a certain formation in that being, taking that personal, that being then took birth in that particular realm. So, you see how the formations really are what determine how you attach to that formation. If there’s attachment to that formation, that’s going to cause rebirth. And it depends upon the quality of that formation, meaning, if it’s a wholesome or an unwholesome formation, which is usually prevalent, it depends on the amount of unwholesome actions or wholesome actions you’ve taken.
In other words, if a being is someone who is consistently wholesome in their actions, in their speech, they will have primarily wholesome activities, they will have primarily wholesome formations. Therefore, they will take rebirth in wholesome realms. Realms where the fruits of their kamma can be experienced. So, it is always through formations to understand how rebirth takes place in different realms, as well as how rebirth takes place in one’s own case, through one’s own formations that one inherited in this life.