He trains himself to breathe in sensitive to mental processes, and to breathe out sensitive to mental processes.
(Cittasaṅkhārapaṭisaṃvedī assasissāmīti sikkhati. Cittasaṅkhārapaṭisaṃvedī passasissāmīti sikkhati.)
He trains himself to breathe in sensitive to the mind, and to breathe out sensitive to the mind.
(Cittapaṭisaṃvedi assasissāmīti sikkhati. Cittapaṭisaṃvedī passasissāmīti sikkhati.)
In MN 44 we find an explanation for cittasaṅkhāra:
"Now, lady, what are fabrications?"
"Perceptions & feelings are mental fabrications." (Thanissaro trans.)
(Saññā ca vedanā ca cittasaṅkhāroti.)
The Visuddhimagga says (VIII, 229ff):
The remaining [three] clauses should be understood in the same
way as to meaning; but there is this difference here.
The experiencing of bliss must be understood to be through three jhanas, and that of the mental formation through four. The mental formation consists of the two aggregates of feeling and perception. And in the case of the clause, experiencing bliss, it is said in the Paþisambhidá in order to show the plane of insight here [as well]: “‘Bliss’: there are two kinds of bliss, bodily bliss and mental bliss” (Paþis I 188). Tranquilizing the mental formation: tranquilizing the gross mental formation; stopping it, is the meaning. And this should be understood in detail in the same way as given under the bodily formation (see §§176–85). Here, moreover, in the “happiness” clause feeling [which is actually being contemplated in this tetrad] is stated under the heading of “happiness” [which is a formation] but in the “bliss” clause feeling is stated in its own form. In the two “mental-formation” clauses the feeling is that [necessarily] associated with perception because of the words, “Perception and feeling belong to the mind, these things being bound up with the mind are mental formations” (Paþis I 188). So this tetrad should be understood to deal with contemplation of feeling.
(ix) In the third tetrad the experiencing of the [manner of] consciousness must
be understood to be through four jhanas
Ven. Buddhadasa  explains that second tetrad as concerned specifically with pīti and sukha whereas the third is concerned with citta in a broader and deeper way.
 - Mindfulness With Breathing: A Manual for Serious Beginners