1. I think Ajahn is describing a kind of samadhi that he has cultivated.
– There are only four jhana that the Buddha described. In Abhidhamma the second jhana is split into two, so the number becomes five. That is just a matter of details.
– But there can be billions of samadhi. Therein lies the danger. Depending on what one is cultivating, one can get into different states of the mind. It seems to me that Ajahn has cultivated an “asañña samadhi” where he does not perceive anything. But, of course, I would not know exactly what it is.
– It is interesting to note that the experience of the lay disciple at the end of your comment seems to be also the same.
– They could be cultivating some kind of “asañña samadhi” which can be dangerous.
– P.S. It is only in nirodha samapatti (of an Arahant) that one becomes totally unaware of “this world”. That is NOT an “asañña samadhi” because one gets into that samapatti by systematically going up the ladder of jhana, getting to the highest arupavacara state (Nevasaññānāsaññāyatana) and then entering nirodha samapatti.
2. One enters a jhana knowingly, and one stays in a jhana knowingly.
– One can think very clearly in a jhana (even better in a samapatti). That is the advantage of a jhana.
– If one knows what one is doing (and realizes the unfruitfulness of jhana in the end), a jhana provides the best environment for “insight meditation” (vipassana).
3. The jhanic experience is described in great detail in the “Sāmaññaphala Sutta (DN 2)“
– I am not sure how good that translation is. I have not read it, at least recently.
– My explanation of the sutta is in the post, “Jhānic Experience in Detail – Sāmaññaphala Sutta (DN 2)”
4. That sutta also provides a good background as well. In it, the Buddha describes to a King the benefits of the life of a bhikkhu (“The Fruits of the Ascetic Life” as the title of the English translation there correctly states).
– In the first part of the sutta, is the important background of a person giving up the lay life and becoming a bhikkhu. As I have explained in the newest post, “Rāga and Jhāna – Two Commonly Misunderstood Words” it is impossible for a layperson to cultivate Ariya jhana, unless becoming an “Anagārika.”
– The detailed description of the jhanic experience is toward the end of the sutta starting in section 126.96.36.199. Paṭhamajhāna (First Absorption)