As regards jhānacitta, jhanacittas do not have as their object, visible object, sound, or any other sense-impression. Jhānacittas arise in a process of cittas experiencing a meditation subject through the mind-door. In this process there are first kāmāvacara cittas which experience the meditation subject and then, in that same process, the jhānacitta arises. The process is as follows:
} mano-dvārāvajjana-citta or mind-door-adverting-consciousness
} parikamma or preparatory consciousness
kāmāvacara cittas } upacāra, which means: proximatory or access
} anuloma or adaptation
} gotrabhū, which means: that which overcomes the
sense-sphere, or 'change of lineage'
jhānacitta } appanā or absorption (the moment of citta
which attains jhāna)
For some, 'parikamma' (preparatory consciousness) is not necessary, and in this case there are, after the mind-door-adverting-consciousness, only three kāmāvacara cittas arising, instead of four, before the jhānacitta arises. Gotrabhu (which 'overcomes' the sense-sphere) is the last citta in that process which is kāmāvacara citta.
In the 'Visuddhimagga' (IV, 74) we can read about the process of cittas in which jhāna occurs for the first time. The 'Visuddhimagga' (IV, 78) states that only one single moment of jhānacitta arises, which is then succeeded by the bhavanga-citta (life-continuum). After that there is a process of kāmāvacara cittas, reviewing, through the mind-door, the jhāna which has just occurred. Further on (IV, 123 ff. ) we read that absorption can 'last' only when it is absolutely purified of states which obstruct concentration. One must first completely suppress lust by reviewing the dangers of sense desires and also suppress the other 'hindrances'.
Jhānacittas are kusala kamma of a high degree. When jhāna has been attained the hindrances of sensuous desire, ill-will, sloth and torpor, restlessness and worry, and doubt are temporarily eliminated. Thus one is truly calm, at least at that moment.