Ānāpānasati (āna-apāna-sati, a Pāli word) means "attention to the breath." In Theravada Buddhism, the breath is the most common object of meditation when it comes to calming the mind, through the practice of "samatha bhavana" meditation.
The breath is always present in every living being, and this phenomenon is sometimes conscious, sometimes not, which seems to be the main reasons for the popularity of this practice.
Attention to the breath is also popular in the practice of "Vipassana" meditation. Mahasi Sayadaw, for example, uses attention to the movements of the abdomen due to the breath as a starting point for vipassana meditation. It can lead to a state of absorption (jhāna).
The ānāpānasati sutta, 118th sutta of the Majjhima-Nikâya of the Pāli canon, is one of the most popular among Theravadins. It describes the practice of mindfulness throughout the day, as well as a 16-point practice of ānāpānasati, blending the practice of concentration and vipassana and leading to the attainment of nirvāna.
The sixteen stages-or contemplations-of ānāpānasati are divided into four groups of four.
The first four stages focus the mind on the breath, which is the "conditioning of the body" (pāli: kāya-sankhāra).
The second tetrad is a concentration on the sensations (vedanā), which are the "conditioning of the mind" (pāli: citta-sankhāra).
The third tetrad is directed on the mind itself (pāli: citta), and the fourth on "the truth" (pāli: dhamma).
A ānāpānasati meditation session should progress through the stages in order, beginning with the first, whether or not the practitioner has performed all the stages in a previous session.
According to the Theravadin monk Ajahn Brahmavamso , the first twelve steps are instructions for jhana and the last four are instructions for what to do next. The first two steps are about observing the breath (not controlling it), such as comparing the length of the inhalation and exhalation.
In the third step, the whole "body of breath" is felt: the attention is exclusively directed to the breath, in all its nuances and modalities. In the fourth stage, the breath becomes calm and more subtle, the attention is focused on it at every moment without trying to recognize if it is an inhalation or an exhalation.
In the fifth and sixth stages, joy and happiness appear naturally and gradually: the mental energy passes totally into observation rather than action.
In the seventh stage, the breath seems to disappear, it is known only as a mental object (citta-sankhara), and the five sense bases are abandoned. The eighth stage is designed to counteract the fear or excitement that may arise at this stage.
In the ninth step, the mind is felt through nimitta (a characteristic sign, such as a very bright light or other sensation). The tenth and eleventh steps increase the brightness and stability of the nimitta: one can focus on the center of the nimitta to increase its brightness, or ignore it temporarily and return to the breath.
The eleventh step allows to sustain the attention on the nimitta, to make it stable. The twelfth step ("vimocayam cittam": "freeing the mind") is the entry into jhana, by absorption in the nimitta: it is not the result of an action, it happens as the natural result of the abandonment of any action.
The mind is now free, free from the body and the five senses: one no longer feels anything related to the body. The last 4 steps follow the "letting go" of jhana, by examining the experience of not-self that has taken place.
Prajñānanda's interpretation is quite different, as it is more oriented towards Prajnaparamita (which may seem paradoxical for a text that is usually attached to Theravada): the four tetrads are independent of each other, and the twelfth step should lead to the Penetrating View, vipassana.
The objective is not necessarily jhana ("to arrive at the unconditioned, there is no need for conditions", so vipassana does not require jhana), but rather "recollecting alertness" which is exercised in the three psychic modes, subconscious, conscious and superconscious, and access to transcendent knowledge (prajñā).
The first tetrad achieves the tranquilization of the body (or bodies). The second tetrad leads to dhyāna, tranquilization of the psyche, access to the "subtle phenomenal".
In the third tetrad the psyche is without activity, completely still, and transcendent knowledge manifests and develops; the liberation referred to in step twelve is the liberation from subconscious purulences (asravas). Once the psyche is still and alert, the fourth tetrad leads to transcendent knowledge and gives access to the Absolute.
This tetrad should not be interpreted as "attention to mental objects", since "dharma" refers to everything that is supported, all phenomena (not only mental objects).
In the Theravāda tradition
According to several teachers of Theravāda Buddhism, the practice of ānāpānassati alone is sufficient to eliminate the impurities of the mind (Kilesa) and to awaken.
Robert Bischoff, translator of Webu Sayadaw, reports that Webu Sayadaw said of ānāpānassati, "This is the shortcut to Nibbana, anyone can use it. It stands up to investigation and it's in accordance with the Buddha's teachings as preserved in the scriptures. It is the direct path to Nibbana.