[accelieda]

To my ears, the translation of the samuppada as "arising motion" is still quite removed from "occurring simultaneously",

Some things arise (based on their causes & conditions) quickly. Other things arise slowly. The man here is having difficulties due to a lack of comprehension. The man here is caught up in philosophy and projecting that philosophical outlook onto Buddhadasa. To break this rigid kind of thinking & conditioning, it is best to quote another teacher:

We don't understand the Dhamma and so we don't understand these sankharas; we take them to be ourselves, as belonging to us or belonging to others. This gives rise to clinging. When clinging arises, "becoming" follows on. Once becoming arises, then there is birth. Once there is birth, then old age, sickness, death... the whole mass of suffering arises. This is the Paticcasamuppada. 9 We say ignorance gives rise to volitional activities, they give rise to consciousness and so on. All these things are simply events in mind. When we come into contact with something we don't like, if we don't have mindfulness, ignorance is there. Suffering arises straight away. But the mind passes through these changes so rapidly that we can't keep up with them. It's the same as when you fall from a tree. Before you know it — "Thud!" — you've hit the ground. Actually you've passed many branches and twigs on the way but you couldn't count them, you couldn't remember them as you passed them. You just fall, and then "Thud!"

Ajahn Chah It's likewise with the teaching of dependent origination (paticca-samuppāda): deluded understanding (avijjā) is the cause and condition for the arising of volitional kammic formations (sankhāra); which is the cause and condition for the arising of consciousness (viññāna); which is the cause and condition for the arising of mentality and materiality (nāma-rūpa), and so on, just as we've studied in the scriptures. The Buddha separated each ]This is an accurate description of reality, but when this process actually occurs in real life the scholars aren't able to keep up with what's happening. It's like falling from the top of a tree to come crashing down to the ground below. We have no idea how many branches we've passed on the way down. [/b]Similarly, when the mind is suddenly hit by a mental impression, if it delights in it, then it flies off into a good mood. It considers it good without being aware of the chain of conditions that led there. The process takes place in accordance with what is outlined in the theory, but simultaneously it goes beyond the limits of that theory.

Ajahn Chah Unshakeable Peace