[410] How does one train thus: "Experiencing the mental formation (citta-saṅkhāra), I shall breathe in," train thus: "Experiencing the mental formation, I shall breathe out"?
What is the mental formation? Perception and feeling (vedanā), by way of the long in-breath, are mental factors (cetasika); these dhammas, being bound up with the mind (citta), are mental formations. Perception and feeling, by way of the long out-breath, are mental factors; these dhammas, being bound up with the mind, are mental formations. … Perception and feeling, by way of experiencing happiness on the in-breath, by way of experiencing happiness on the out-breath, are mental factors; these dhammas, being bound up with the mind, are mental formations. This is the mental formation.
How are those mental formations known? For one who understands the one-pointedness and non-distraction of the mind by way of the long in-breath, mindfulness is established; by that mindfulness and that knowledge (ñāṇa) those mental formations are known. For one who understands the one-pointedness and non-distraction of the mind by way of the long out-breath, mindfulness is established; by that mindfulness and that knowledge those mental formations are known. … For one realizing what is to be realized, those mental formations are known. Thus are those mental formations known.
By way of the in-breaths and out-breaths of one experiencing the mental formation, feeling is the establishment (upaṭṭhāna); mindfulness is the knowledge of contemplation. Feeling is the establishment, not mindfulness; mindfulness is both the establishment and the mindfulness itself. By that mindfulness and that knowledge one contemplates that feeling. Therefore it is said: "the development of the establishment of mindfulness (satipaṭṭhāna) by contemplating feelings in feelings."
"Contemplates": how does one contemplate that feeling? One contemplates it as impermanent … thus does one contemplate that feeling.
"Development" (bhāvanā): there are four kinds of development … development in the sense of repeated practice. The in-breaths and out-breaths of one experiencing the mental formation are purity of virtue (sīlavisuddhi) in the sense of restraint … For one who understands the one-pointedness and non-distraction of the mind by way of the in-breaths and out-breaths of one experiencing the mental formation … understanding, one brings the faculties (indriya) together; therefore it is said, "and he penetrates that which has peace as its meaning."
English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.