Visuddhimagga: XI: Concentrating (samaadaha.m) the (manner of)
consciousness:"evenly
(samam) placing (adahanto) the mind, evenly putting it on its object by
means
of the first jhana and so on. Or alternatively when, having entered upon
those jhanas and emerged from them, he comprehends with insight the
consciousness associated with the jhana as liable to destruction and fall,
then at the actual time of insight momentary unification of the mind arises
through the penetration of the characteristics...
Nina: When the yogavacara, the practitioner, concentrates on the meditation
subject, in this case, breath, he needs right understanding and also samadhi
that concentrates again and again and again, so that it can become access
concentration and attainment concentration when he attains jhana. When he
can have jhanacitta for many moments, there are no cittas of the sense
sphere and no bhavangacittas in between. His concentration on the meditation
subject is stable. The word evenly applies to jhana, when there is no
disturbance by sense impressions. When he emerges from jhana and he can
develop insight, there is momentray concentration with the citta that
realizes the happiness of jhana as a dhamma arising and falling away. The
Visuddhinmagga speaks about <momentary unification of the mind>.
The Vis. I, note 3 explains that no insight comes about without momentary
concentration.