Patisambhidhamagga has an explaination. Unfortunately there is no english translation available in suttacentral.
Passambhayaṃ kāyasaṅkhāraṃ
Kathaṃ ‘‘passambhayaṃ kāyasaṅkhāraṃ assasissāmī’’ti sikkhati, ‘‘passambhayaṃ kāyasaṅkhāraṃ passasissāmī’’ti sikkhati? Katamo kāyasaṅkhāro?
How does he train as, “I will breathe in calming the bodily-fabrication”. How does he train as, “I will breathe out calming the bodily-fabrication”. (passambhayaṃ kāyasaṅkhāraṃ)
What is bodily-fabrication?
The long in-breaths are bodily. These phenomena that are bound to the body are bodily-fabrication. He trains, (by) calming, dissolving, tranquillizing those bodily-fabrications.
The long in-breaths are bodily.These phenomena that are bound to the body are bodily-fabrications. He trains (by) calming, dissolving, tranquillizing those bodily-fabrications. (Also) the short in-breaths and short out-breaths.
The in-breaths, experiencing the whole body and the out-breaths, experiencing the whole body are bodily. These phenomena that are bound to the body are bodily-fabrications. He trains, (by) calming, dissolving, tranquillizing those bodily-fabrications.
Those kind of bodily-fabrications such as bending forward and backward of the body, contracting and stretching, moving, throbbing, trembling, quaking – he trains as, “I will breath-in calming (those kind of) bodily-fabrication”; he trains as, “I will breath out calming (those kind of) bodily-fabrication”.
Those kind of bodily-fabrications such as not bending forward and backward of the body, not contracting and stretching, not moving, not throbbing, not trembling, not quaking, peaceful, subtle – he trains as, “I will breath-in calming (those kind of) bodily-fabrication”; he trains as, “I will breath out calming (those kind of) bodily-fabrication”.
Thus indeed, he trains as, “I will breathe in calming the bodily-fabrication”; he trains as, “I will breathe out calming the bodily-fabrication”. When it is so (when calming thus), the citta which has the breath-as-object does not arise; the subtle in and out breaths does not arise; the mindfulness of in and out breaths does not arise; the concentration based on the mindfulness of in and out breaths does not arise; the wise one who is endowed with this does not attain and also does not produce that (‘calming’).
Thus indeed, he trains as, “I will breathe in calming the bodily-fabrication”; he trains as, “I will breathe out calming the bodily-fabrication”.
When it is so (when calming thus), the citta which has the breath-as-object does arise; the subtle in and out breaths does arise; the mindfulness of in and out breaths does arise; the concentration based on the mindfulness of in and out breaths does arise; the wise one who is endowed with this does attain and produce that (‘calming’).
Like what?
Just like when a bronze gong is first struck, coarse sounds arise. When the clear, distinct, sharp, mark of the coarse sound stops, subtler sounds arise after that. When the clear, distinct, sharp mark of the subtle sound stops, the mind which has the object of the mark of the subtle sound arises (next) after that.
In the same way, first coarse in and out breaths arise. When the decipherableness, observability, locatableness of the mark of the coarse breath stops, the subtler sounds arise next. When the decipherableness, observability, locatableness of the mark of the subtle breaths stops, the mind which has the object of the mark of the subtle breath arises next.
When it is so (when calming thus), the citta which has the breath-as-object does arise; the subtle in and out breaths does arise; the mindfulness of in and out breaths does arise; the concentration based on the mindfulness of in and out breaths does arise; the wise one who is endowed with this does attain and produce that (‘calming’).
Passambhayaṃ cittasaṅkhāraṃ
How does he train as, “I will breathe in calming the mental-formation”; he train as, “I will breathe out calming the mental-formation”? (passambhayaṃ cittasaṅkhāraṃ)
What is mental-formation?
The mental factors of perception and sensation, in terms of long in-breath – these phenomena are mind-bound, mental-fabrications. He trains, (by) calming, dissolving, tranquillizing those mental-fabrications.
The mental factors of perception and sensation, in terms of long out-breath – these phenomena are mind-bound, mental-fabrications. He trains, (by) calming, dissolving, tranquillizing those mental-fabrications.
…in terms of breathing in, experiencing the mental-fabrication…
The mental factors of perception and sensation, in terms of out-breath, experiencing mental-fabrication – these phenomena are mind-bound, mental-fabrications. He trains, (by) calming, dissolving, tranquillizing those mental-fabrications.
In and out breaths, calming mental-fabrications is the sensation. The establishing is mindfulness. The contemplating is knowledge. The establishing of ‘sensation’ is not mindfulness. The establishing of mindfulness is only mindfulness. By that mindfulness, by that knowledge, he contemplates that ‘sensation’.
ĀNĀPĀNASSATIKATHĀ