In the case of the verbs assasanto & passasanto, the Sanskrit considers dīgha (दीर्घ dīrgha) and rassa (ह्रस्व hrasva [hras-va]), as adverbs - whose translation is far (lofty) & close (low) - and not long & short (as it would be, in the case of an adjective).
Long & short has been, without fairness, the prefered translation of dīrgha & rassa.
However, in Anapanasati, "paṇidhāya parimukhaṃ satiṃ upaṭṭhapetvā" means (along with a proper lexicography) :
"Intending towards the beginning, and having looked after the obtention (sati).
(see sati & sati : https://justpaste.it/53vyj)
Here the "beginning" means that one should turn his/her mano towards the origin (yoniso manasi-kara).
The origin being where the dhamma "breath" originates. That is to say in the arupa-loka - namely, in the ("lofty") nāmarūpa nidāna; where the (first) coactions (saṅkhārā) of khandhas = dhammas take place. (So ultimately, one should move his/her attention along the arrow #2, https://justpaste.it/1n1ii [down & up] - and deflect path #1).
Note that the first breath's co-action occurs in the saṅkhārā nidāna, but it might not yet be called a dhamma per se. Dhammas seem to be only considered as phenomena directed to, and experienced in, the saḷāyatana nidāna and below.
Then, "ajjhattaṃ vā kāye kāyānupassī viharāhi," whose proper translation is:
Internally, he fetches distinctively the noticeable body (in this case: breath), from the bodies (either lofty/far, or low/close/(satta's) breaths),
becomes much clearer.
Finally, the MN 10's extract might make much more sense:
"In this way he abides fetching distinctively the noticeable body from the bodies internally, or he abides fetching distinctively the noticeable body from the bodies externally, or he abides fetching distinctively the noticeable body from the bodies both internally and externally".
(Breathing in (towards) - Breathing out (away))
आश्वस् āśvas [ā-śvas]
प्राश्वस् prāśvas [prā-śvas]
- towards (the speaker).
√ श्वस् śvas
- to pant [breathe heavily, as if exhausted] (RV.)
- to breathe (MBh.) (also = live)
- to cause to blow or breathe ; to cause heavy breathing (Suśr.)
- to breathe
- inhalation (breathing towards)
- exhalation (breathing away)
Having folded one's legs crosswise, and straightened one's body.
Secluded from sensual pleasures (vivicceva kāmehi) and secluded from unproper states (vivicca akusalehi dhammehi).
One intends towards the beginning [viz. one should turn his/her mano towards the origin (yoniso manasi-kara).
The origin being where the dhamma "breath" originates. That is to say in the arupa-loka - that is in nāmarūpa nidāna)], and having looked after the obtention (sati) [of Citta], just mindful (viz. thinking upon the breath).
One breathes (quite heavily) in, towards the far/lofty (viz. the nāmarūpa nidāna - the arūpa loka), mindful - & one breathes (quite heavily) out, away from the "far/lofty", mindful.
Then, one breathes (quite heavily) in, towards the "close/low" (viz. the saḷāyatana nidāna - the rūpa loka), mindful - & one breathes (quite heavily) out, away from the "close/low" (viz. the saḷāyatana nidāna - the rūpa loka).
It is only in this way, that one can have:
1. an accurate knowledge of the entire body (sabbakāyapaṭisaṃvedī) in step three of Ānāpānasati).
2. and an accurate knowledge of pīti (pītipaṭisaṃvedī) [the mano-like pleasure], & an accurate knowledge of sukha (sukhapaṭisaṃvedī) [the citta-like pleasure] (see SN 35.97), in the following fifth and sixth step of Ānāpānasati.
‘There is this, there is the inferior, there is the superior, and beyond there is an escape from this whole field of perception.’