The key issue is how a hadaya vatthu arises for a given existence. It is explained in Paticca Samuppada (PS):
1. A hadaya vatthu for a future existence arises due to a specific kammic energy created in this life or previous lives. Let us take my hadaya vatthu.
2. When I die, my gandhabba (with hadaya vatthu) will come out of my physical body. Then it will wait in that gandhabba state until pulled into a matching womb, and I will be reborn to have a different human body. That process will continue until the lifetime of my hadaya vatthu is exhausted.
3. At that point, the last citta vithi from this hadaya vatthu will run, which will be focused on an arammana corresponding to the STRONGEST kamma from my past.
– Suppose I had killed another human being. That arammana would give rise to an existence in an apaya (one of the four lowest realms). But my mind WILL NOT grasp that bhava (existence) IF I have attained a magga phala. If I did not have a magga phala, then the mind would AUTOMATICALLY grasp that existence. That is the “upadana paccaya bhava” step in Uppatti PS.
4. I would have killed that human either in this life or a previous life (of course, this is just an example) via (abhi)sankhara that arose due to avijja: “avijja paccaya sankhara” in PS.
5. Of course, I would have done that ONLY IF there was a strong arammana that induced strong greed or anger (with avijja) at that time.
– That is why I have explained that the PS cycle does not start at “avijja paccaya sankhara.” One starts acting with avijja ONLY IF one gets attached to an arammana.
– I have tried to explain that in the recent series of posts: “Paṭicca Samuppāda During a Lifetime.”
– That description is for Idappaccayātā Paṭicca Samuppāda. But the sequence of events is the same for grasping an “uppatti bhava” or a brand new existence in another realm. Idappaccayātā Paṭicca Samuppāda explains “temporary bhava” grasped during a lifetime.
6. The bottom line is that brand new hadaya vatthu can ONLY be created by kammic energy.
– An Arahant’s mind would not grasp any existence (bhava) within the 31 realms. Thus, in Arahant’s last citta vithi, the Uppatti PS STOPS at the “upadana paccaya bhava” step. Thus, the next two steps (“bhava paccaya jati” and ‘jati paccaya jara, marana, soka, ..”) would not arise either. That is the end of suffering in the rebirth process.