Various translators have profound disagreements over how to translate that passage and whether the Commentary has the correct interpretation. I don't see how you'll answer a question that various scholars disagree on by pondering over dictionaries.
Sabbakayapatisamvedi Assasissami... passasissamiti sikkhati... = "Experiencing the whole body I shall breathe in... breathe out, thinking thus, he trains himself." He trains himself with the following idea: I shall breathe in making known, making clear, to myself the beginning, middle, and end of the whole body of breathings in; I shall breathe out making known, making clear, to myself the beginning, middle and end of the whole body of breathings out. And he breathes in and breathes out with consciousness associated with knowledge making known, making clear, to himself the breaths."
"To one bhikkhu, indeed, in the tenuous diffused body of in- breathing or body of out-breathing only the beginning becomes clear; not the middle or the end. He is able to lay hold of only the beginning. In the middle and at the end he is troubled. To another the middle becomes clear and not the beginning or the end. To a third only the end becomes clear; the beginning and the middle do not become clear and he is able only to lay hold of the breath at the end. He is troubled at the beginning and at the middle. To a fourth even all the three stages become clear and he is able to lay hold of all; he is troubled nowhere. For pointing out that this subject of meditation should be developed after the manner of the fourth one, the Master said: Experiencing... He trains himself."
"Since in the earlier way of the practice of this meditation there was nothing else to be done but just breathing in and breathing out, it is said: He thinking, I breathe in... understands... and since thereafter there should be endeavor for bringing about knowledge and so forth, it is said, Experiencing the whole body I shall breathe in."
Passambhayam kayasamkharam assasissamiti... passasissamiti sikkhati = "Calming the activity of the body I shall breathe in... breathe out, thinking thus, he trains himself." He thinks: " I shall breathe in and I shall breathe out, quieting, making smooth, making tranquil and peaceful the activity of the in-and-out-breathing body. And in that way, he trains himself."
"In this connection coarseness, fineness and calm should be understood thus: Without contemplative effort, the body and the mind of this bhikkhu are distressed, coarse. When the body and the mind are coarse, the in-and-out-breathings too are coarse and proceed uncalmly; the nasal aperture becomes inadequate and he has to breathe through the mouth, too. But when the body and the mind are under control then the body and the mind become placid, restful. When these are restful, the breathings proceed so fine that the bhikkhu doubts whether or not the breathings are going on."
"The breathing of a man who runs down from a hill, puts down a heavy burden from his head, and stands still is coarse; his nasal aperture becomes inadequate and he breathes through the mouth, too. But when he rids himself of his fatigue, takes a bath and a drink of water, and puts a wet cloth over his heart and is sitting in the shade, his breathing becomes fine, and he is at a loss to know whether it exists or not. Comparable to that man is the bhikkhu whose breaths become so fine after the taking up of the practice of contemplation that he finds it difficult to say whether he is breathing or not. What is the reason for this? Without taking up the practice of meditation he does not perceive, concentrate on, reflect on, or think over, the question of calming the gross activity of the breathing body, the breaths, but with the practice of meditation he does. Therefore, the activity of the breath-body becomes finer in the time in which meditation is practiced than in the time in which there is no practice. So the men of old said:
"In the agitated mind and body the breath is of the coarsest kind.
In the unexcited body, fully subtle does it wind."
"How does he train himself with the thought: Calming the activity of the body, I shall breathe in... breathe out? What are the activities of the body? Those things of the body of breaths, those things bound up with that body, are the activities of the body. Causing the body-activities to become composed, to become smooth and calm, he trains himself... He trains himself thinking thus: Calming the body-activity by way of (quieting) the bodily activities of bending forwards, sidewards, all over, and backwards, and (by way of the quieting of) the moving, quivering, vibrating, and quaking of the body, I shall breathe in... I shall breathe out. I shall breathe in and I shall breathe out, calming the activity of the body, by way of whatsoever peaceful and fine body-activities of non-bending of the body forwards, sidewards, all over and backwards, of non-moving, non-quivering, non-vibrating, and non-quaking, of the body."
Indeed, to that yogi training in respiration-mindfulness according to the method taught thus: "He, thinking 'I breathe in long,' understands when he is breathing in long... Calming the activity of the body... I breathe out, thinking thus, he trains himself" [digham va assasanto digham assasamiti pajanati... passambhayam kayasankharam passasissamiti sikkhati], the four absorptions [cattari jhanani] arise in the respiration sign [assasapassasanimitte uppajjanti].
In the respiration sign = In the reflex image [patibhaga nimitta].
Having emerged from the absorption, he lays hold of either the respiration body or the factors of absorption.
There the meditating worker in respiration [assasapassasa kammika] examines the body (rupa) thinking thus: Supported by what is respiration? Supported by the basis [vatthunissita]. The basis is the coarse body [karajja kaya]. The coarse body is composed of the Four Great Primaries and the corporeality derived from these [cattari mahabhutani upadarupañca].
The worker in respiration examines the respiration while devoting himself to the development of insight through the means of corporeality.
The basis, namely, the coarse body, is where the mind and mental characteristics occur.
Thereupon, he, the worker in respiration, cognizes the mind (nama) in the pentad of mental concomitants beginning with sense-impression.
The first beginning with sense-impression are sense-impression, feeling, perception, volition, and consciousness. They are taken here as representative of mind.
The worker in respiration examines the mind and the body, sees the Dependent Origination of ignorance and so forth, and concluding that this mind and this body are bare conditions, and things produced from conditions, and that besides these there is neither a living being nor a person, becomes to that extent a person who transcends doubt.
[19.] In the explanation of the contemplation on breathing, the passage beginning with "When breathing in long, how does he understand, 'I breathe in long'" and ending with the words "non-quaking of the body," consists of extracts from pages 272-277 of the Visuddhi Magga, Part 1. P.T.S. Edition.