This is a way of understanding non-duality. Because consciousness is infinite, there is the possibility of self-ignorance in individual living beings. This is due to āvaraṇa śakti (the concealing power of māyā), fragmented and set in motion by its quality vikṣepa śakti (the creative, projecting power of māyā). But ignorance is apparent (vivarta), not real. In reality, everything is pure knowledge, pure consciousness. In reflected form, this becomes the relative knowledge of the world.

If one thinks that the objects of body, mind, and world are real, then one is ignorant that it is all, in reality, an ocean of consciousness, which is oneself. Correct self-knowledge is either on or off. That is why Bhagavad Gītā 2.69 states:

yā niśā sarvabhūtānāṁ tasyāṁ jāgarti saṁyamī
yasyāṁ jāgrati bhūtāni sā niśā paśyato muneḥ

What is night for all beings, in that the wise (saṁyamī) is awake;
and where all beings are awake, that is night for the 'seeing' wise (muni).

Therefore, on an individual level, one is either in knowledge or in ignorance. Both simultaneously are not possible. It is either/or, not and/and. Two possibilities of the same coin of reality. Ignorance is, in reality, also awareness. Jñāna śakti, with its revealing quality of sattva, is the clear force that makes me see this when I hear the teacher recite the propositions of the Upanishads.

I just wrote at the same time. Whether one can then relapse into ignorance (the on-and-off "blinking," as James Swartz calls it) has been a point of discussion in Vedānta after Śaṅkara. Sureśvara (vārttika prasthāna), Śaṅkara's direct successor in the Śṛṅgeri Maṭha (Southern Śaṅkara temple), and Padmapāda/Prakāśātman (vivaraṇa prasthāna) argue that śravaṇa (listening to the guru explaining the mahā-vākyas of the scriptures) is sufficient. Vācaspati Miśra (bhāmatī prasthāna), however, states that nididhyāsana (and even samādhi) is necessary for knowledge.

The truth lies somewhere in between. It is true that knowledge must come from outside to remove individual ignorance, and this can only be śravaṇa. Yet, knowledge in the mind can easily shift into ignorance, as any self-practitioner will attest. Therefore, one must listen again and again, reason it through (māna), and allow the knowledge to endure as life's issues manifest. However, knowledge occurs in śravaṇa. Once the seed is planted in a qualified person, they will have acquired a taste of freedom and will remain with it.

Therefore, modern Vedānta teachers, following the example of Ādi Śaṅkara Bhagavatpādaḥ himself, utilize methodologies (prakriyās) from all three schools/sources/systems (prasthānas).

Knowledge in the mind is therefore either the knowledge that freed someone from ignorance, or it is ignorance. This is all in the relative realm. Absolute pure knowledge (jñapti, vijñāna) is beyond this, or precedes it. It is consciousness itself, that which is never truly transformed into anything (apariṇāma or avikāra).

A being manifested because there was mūlāvidyā, potential ignorance due to previous karma.

The knowing being no longer manifests after its prārabdha (or ārabdha, if you prefer), the karma that has already begun to replay a lifetime, has been exhausted.