Ordinarily, attention tends to be absorbed in this or that. Or rather, it tries to be absorbed, it seeks absorption, it craves for it. Sensual pleasures provide perhaps the simplest and most apparent example. The greater the pleasure, the stronger the absorption. But attention can also seek absorption into more complex contents or situations, layered with meanings, scaffolded by social and cultural structures.
Absorption can be sustained on the same object only for relatively short periods. Seconds? Minutes? Hours? Maybe days? Eventually, absorption is always uncomfortable. If it is too short and ends sooner than one wants, then it leaves the sense of something interrupted and to restore again, one is separated from what one liked. If it lasts too long, then either one starts feeling caged in the experience that is now established, or that same experience changes and becomes otherwise, so that one is now yoked with what one does not want. In either case, absorption is somehow disrupted and (ordinarily) one has to start again, find a new situation, or renew the one in which one is. Do something, and repeat.
In engaging with this endless cycle, all the energies of attention are focused on contents, whether they are suitable or not, whether they should be pursued or ignored. But the background context in which this whole experience unfolds remains unexamined. Breath recollection is one way of reversing this attitude.
The core insights that underpin the practice of breath recollection can be summarized quite simply:
(1) Any content of experience (and any constituent of experience) is uncertain and subject to change, regardless of what anybody wants or wishes about this fact. Absorption in any content is thus doomed to be disrupted. Or rather, seeking absorption reveals itself for what it is: a strategy devised to deliberately avoiding and ignoring the glaring evidence of uncertainty, which is implicitly sensed as scary and threatening.
(2) To face this fear, one needs first to stop being absorbed in contents and look at the context that underpins their appearing. This context is very simple, as simple as the sheer fact that there is a body present. Because there is body, there is experience. To make this more apparent, take the breath: thanks to the breath, there is this or that experience. If there was no breath, one would be simply dead. The breath is not the cause of this or that content, but the general condition or ground for any content to manifest. In this sense, anything else is secondary with respect to the breath, and the breath is the overall context of experience. By deliberately sustaining this recollection, and by establishing the perspective that it entails (the breath first, anything else second), attention is no longer absorbed in any content, and yet it remains attuned to the context in which anything else appears. Open, relaxed, bright.
(3) The context in which anything else appears (the breath, the body) is uncertain, changeable, beyond one's ultimate control, unsuitable for fully settling in and appropriating it. This is what seems scary from an ordinary perspective. But the fear is there only because ordinarily this uncertainty is not examined, only covered up. If one manages to look more closely into it, it will become apparent that there is nothing scary in it. Uncertainty means that whatever appears is just that, something that appears, it belongs to nobody, it is naturally impersonal and free. This applies to the very constituents of experience, to the breath, the body, and even to consciousness. When this realization is established, it becomes a source of peace and relief. Fear presupposes that someone has to defend something that has been appropriated as 'mine.' Without appropriation there is nothing to defend, hence, nothing to fear. Change still happens, contents are still uncertain, but no further consequence follows. Uncertainty is just uncertainty, nothing else.
In actual practice, these points are encapsulated in the first tetrad of the sequence of breath recollection (MN 118). Having preliminary established a degree of morality, sense restraint, and having abandoned the coarser manifestations of the hindrances, one tunes into the breath, allowing attention to rest in, and recollect the breath as the background scenario within which anything else happens. One does not focus on (or try to be absorbed in) the breath, but simply recognizes that anything else is secondary. By not being absorbed in any other content, the breath emerges by itself as the context of all that happens, or else is left alone in the middle of the scene by the progressive fading away of all other concerns.
Then, one observes how the breath is conditioned itself by the body, and vice versa. The whole body is the context of the breath, and their mutual relation immediately reveals the uncertainty and precarity of the whole structure. It's enough to miss a few breaths for the body to die. But drawing the right inference from this, one can acknowledge that all that this means is that neither the body nor the breath is anything that 'I own' or that is 'mine' properly. 'I am' here because there is this breathing body, 'I' too am secondary to it. The breath (the body) is naturally impersonal, naturally belonging to nobody's. And whatever uncertainty might go with it, that's nobody's problem. Nothing to fear.