In the third stage of this meditation practice — the Mindfulness of Breathing — we’re usually aware of quite a large area of the sensation associated with the breath. We may have been focusing primarily on the belly, or the chest, or the sensations in the head and throat. You may even have been aware of all of this sensation.
In the fourth stage however we’re following a very small area of sensation — just the sensations on the rims of the nostrils.
When I first learned this practice I was unsure how to move from one stage to another. I’d simply stop — sometimes abruptly — doing one stage and start — equally abruptly — doing the next. I think that’s a common approach, and I think it’s unhelpful since it brings a dollop of unmindfulness into a mindfulness practice.
Now I like to make a smooth transition from one stage to the other, in order to maintain more of a sense of continuity, and to bring more elegance into my mind.
I do this by narrowing my focus with every breath. Over a series of perhaps seven or eight breaths, I’ll start to narrow down my focus, “homing” in on the sensations on the rims of the nostrils.
In the first breath I might be focused on the whole breath, right down to the belly, on the next perhaps on the whole of the chest, throat, and head. Then just the upper chest, throat, and head. Then the throat and head. Then the head. Then just in the nostrils, and then the tips of the nostrils.
Gradually homing in in this way brings more elegance and smoothness and so helps the stages flow together better.