Hey JOrrit,
I will try to convey my understanding of this practice as it has been taught to me in my tradition.
We think we know our bodies, we think we have control over them. If I give the command arm move, my arm moves, if I give the command eye blink, it blinks. But what about my heart, if I give the command heart stop beating, it does not obey this order it continues to beat, if I command my stomach to stop digesting food it does not obey, it continues to digest food. The breath however works both ways, if I give the command take a deep breath, I take a deep breath, if i give the command take a shallow breath, I take a shallow breath, I can even stop breathing for a short period of time. But when I stop commanding the breath it goes back to its normal rhythm, it functions without the need to be commanded. Because the breath functions in this manner it acts like a bridge between these two levels of consciousness.
Now in my practice, I use the breath at the nostrils to develope concentration and a sharp mind. If my mind is dull, I begin just following the in breath and out breath as my mind concentrates and sharpens, I can feel the touch of breath, I can determine which nostril the breath is entering and exiting sometimes its from both, with further concentration the atmospheric temperatures of the breath becomes aparent, I notice the cool in breath and notice the warmer exhale. As concentration deepens the breath becomes very subtle and faint but I notice its continuity, there is no break like a constant waterfall from the nostrils to the upper lip. Even deeper a subtle sensation begins to become apparent on the tip of my upper lip, like a pulsing sensation. At this point I transition from anapana to vipassana, I switch my awareness away from the breath and give all importance to the sensations in this small area between the upper lip and nostrils.
Now, sensations(inner body energy) act like another bridge, a bridge between the dimension of form, and the dimension of the formless(nibanna). Observing these bodily sensations, we begin to penetrate past the aparent gross solidified sensations, we begin to witness these solidified sensations breaking up into very fast moving constantly arising and pasing away subtle sensations, a flow of energy.
Our job as meditators is to simply watch this process of mind turning into aparent solid matter, breaking up and changing back to mind without reacting. Like a bonfire burning, we simply watch the fire burn out without adding any new wood to the fire, it will take some time but eventually the fire will burn out. The trouble is that this is easy to understand but difficult to practice. Our old habit of the mind is to react with craving towards these pleasant subtle sensations and with aversion towards these gross intensified solidified sensations, we are constantly throwing more wood on the fire.
It is very important that we develope the ability to feel these subtle sensations, we need to work at the deepest level of mind to cut the roots of our defilements.