I haven’t specified a particular point in the body at which to place attention in order to be mindful of breathing.
This is because the Buddha didn’t leave any instruction to do so. Nor does ‘concentration’ (samādhi) mean holding attention onto a particular point – samādhi is a state that the mind enters into as it settles down and feels at ease. Such inner stability is the result of sustained mindfulness.
Discarding any hindrance to mindfulness is a major point.
To this end, moving attention slowly around the body through placing-and- sensing is a useful exercise for developing agility and precision of focus, so that the mind doesn’t get bogged down with, or spin out into, thoughts, passions and agitation.
Also, when meditating on breathing, a focus can be placed on any part of the body that is affected by the flow of breath – nostrils, back of the throat, centre of the chest, or diaphragm are obvious examples. Conducted in the right way, and gradually widening the focus to include more of the felt body, such exercises will support right effort and right mindfulness and help to dislodge obsessive thoughts.
However, as the sutta suggests, the main gathering point is the upright axis.
Slowly sweeping attention along that axis (like stroking a cat) should smooth out any distortions and freshen up numb places in that axis; over time, the energy of breathing will flow in line with that and open through a widening area of your body.
I would caution against placing attention on a narrow point, unless it calls for your kind attention. Experience indicates that many people these days live in a contracted bodily condition that goes along with pressurized mind-states.
When the default of attention is to be narrow and driven, and when the embodied energies are tense or numb (to the degree that a comfortable experience of the body as an entirety is rare or unknown), the instruction to focus on one point in the body can be problematic.
It’s also good to remember that the meditation is on breathing, not on the breath. So whereas focusing on the breath will generally mean focusing on sensations in the body where the air strikes it, breathing is a process that affects the body’s energy system – which connects directly to the citta as ‘heart’.
Moreover, the instructions mention being fully sensitive to the entire body; in which case focusing on one particular point would only be relevant in order to bring that area in line with the whole body. In which case I’d recommend bearing that point in mind and steadily widening the focus to cover your physical (and felt) form.
This reflection by Ajahn Sucitto is from the book, Breathing Like a Buddha, “Get to the Point,” (pdf) pp. 71-73.