In the Pure Dhamma site it is mentioned that Anapansatti is watchfulness on arising of bad thoughts, impulses of bad habits, etc. -- and when noticed, not to get carried away with them. This weakens the fourth functionality of mind of reacting , hence if not reacted upon, no sankhara is formed, and the flow of consciousness in a slight way is setbacked.
Of course there are innumerable sankharas piled up in innumerable lives but even though they may be innumerable, they can be sided by or eradicated. It is only the habit/conditioning of mind that is responsible for enhancing the sankharas and consequently strengthening its habit/conditioning.
For example, suppose there is a person having no greed for money: then even if millions are kept in front of him, he will not react so as to have it; and then another man, with greed inbuilt for money, will react upon it. So here the habit of mind for reacting driven by the greed comes into play.
According to Buddha there are five panchdoshas, greed (kam or lobha), anger (krodh), hatred etc. Greed/kam/lobha are based upon craving, and anger/hatred is based upon aversion. These two are basic weapons of mind to strengthen its habit of reacting.
In breath, anapan satti the awareness or mind is concentrated upon in/out breath i.e it is focused in one kasina (object for concentration), which normally otherwise is scattered in receiving inputs from five sensory bases.
Having done so, first of all it gets rested from wandering. Now the breath is such type of kasina that it neither develops aversion or craving towards it and alas the mind is of habit to avert or crave. Hence mindfulness in breath slowly weakens the habit of the mind and we experience that, if not repeated, the habit diminishes and finally extinguishes.
In my opinion this is everything all about the practice of anapan, with stopping of additions/inputs of reactions, will definitely leave unto the path of nibbana. It's the habit of mind driven by either aversion or craving responsible to take next birth to satisfy itself.
Regards, with best luck to tread on path without any doubt, and understanding the hindrances given by Buddha while treading on the path.
Anchal Kate, India
To add more--The mind is ingrained with habit/conditioning of reacting abiding to craving /aversion since many lives. Hence stilling it, slowly weakening its habit, may take much time: perhaps many lives. In our normal life we experience that if in anything there are no fruits as desired by mind, our mind has no interest in it: it never gets agitated or reacts on that thing further. It implies that whatever mind learns from experience is inbuilt in it, and based upon that experiance it either reacts or not react .
Hence in kayanupasana i.e mindfulness upon bodily sensations without reacting to it, the fact that the sensations arise and passes away is realized by mind upon experiential level to give rise to the understanding of anitya.
If it is anitya what to react upon? Now another e.g there are lot many cultures, teachings, elders, schoolings, which goes on teaching virtues and true values but then where everything goes when we observe so much chaos in spite of the said teachings of virtues. The reason is, all this teachings is given to the conscious mind whose all actions are superficial ,this is only 4 percent or so. The subconscious which is almost 97 percent or so is the captain of the ship.
All the shoutings of teachings does not reach up to it. but if it reaches somehow then the results can be seen. Then on what input signals it works, it works on the inputs of sensations, and with weapons of aversion or craving built in it, it reacts without even knowing to our conscious. The so called conscious learnt degrees of education, mannerisms, cultures, religious upbringing is shocked when it sees the reaction shooted out of it. This is known phenomenon encountered in our daily life.
So the subconscious is 24/7 busy in reacting to the sensations, the only language it understands, without knowing to our conscious. Hence observation of sensations and remaining neutral to it by not averting/craving nullifies the sensation produced on receipt of input from any of the senses: this is what is done in vipassana ,always to remain alert and vigilant. Among many saints there is one saint named Tukaram from Maharashtra whose one of many quotes says, "day and night we are on war duty". In the context of vipassana this is really understood what he wants to say. Hence anapan is okay up to stilling of conscious mind but vipassana is needed to weaken the captain which is subconscious. If at all some day some one explains exactly why and how the sensations are produced upon contact through the senses, then having rightly understood everybody will practice it to eradicate the sankharas and ultimately upon exhaustion of all sankharas attaining nibbana.
NIBBANA is nothing but eradication of all sankharas/conditioning. There is nothing left to take rebirth for. Even if we forget about how to avoid rebirth because many among us does not have understood on subconsciuos level the futility of life . it is only at conscious level arised due to some sort of depressions due to the agonies of life. If the agonies are removed again we will be jumping and dancing in our life turmoil. The deep inside constant agitation at subconscious level does not keep us in happy /satisfied state. practising anapan/vipassana can definitely reduce the agitations which is of utmost importance in our present day lives. The path of avoiding rebirth/attaining nibbana can only be trod once at experiential level understands the impermanance/anityabodh.
This all above has been derived after attending vipassana 10 days course of goenkagurujiat igatpuriand with strong belief , book "art of living"by william harts and personal experiance of stillness of mind on practice of anapan and development of belief on this scientific discovery by buddha as our belief in law of gravitation by newton