And the Blessed One entered the first jhana. Rising from the first jhana, he entered the second jhana. Rising from the second jhana, he entered the third jhana. Rising from the third jhana, he entered the fourth jhana. And rising out of the fourth jhana, he entered the sphere of infinite space. Rising from the attainment of the sphere of infinite space, he entered the sphere of infinite consciousness. Rising from the attainment of the sphere of infinite consciousness, he entered the sphere of nothingness. Rising from the attainment of the sphere of nothingness, he entered the sphere of neither-perception-nor-non-perception. And rising out of the attainment of the sphere of neither-perception-nor-non-perception, he attained to the cessation of perception and feeling.
And the Venerable Ananda spoke to the Venerable Anuruddha, saying: “Venerable Anuruddha, the Blessed One has passed away.” “No, friend Ananda, the Blessed One has not passed away. He has entered the state of the cessation of perception and feeling.”
Then the Blessed One, rising from the cessation of perception and feeling, entered the sphere of neither-perception-nor-non-perception. Rising from the attainment of the sphere of neither-perception-nor-non-perception, he entered the sphere of nothingness. Rising from the attainment of the sphere of nothingness, he entered the sphere of infinite consciousness. Rising from the attainment of the sphere of infinite consciousness, he entered the sphere of infinite space. Rising from the attainment of the sphere of infinite space, he entered the fourth jhana. Rising from the fourth jhana, he entered the third jhana. Rising from the third jhana, he entered the second jhana. Rising from the second jhana, he entered the first jhana.
Rising from the first jhana, he entered the second jhana. Rising from the second jhana, he entered the third jhana. Rising from the third jhana, he entered the fourth jhana. And, rising from the fourth jhana, the Blessed One immediately passed away.
(“Maha-parinibbana Sutta: Last Days of the Buddha”, translated from the Pali by sister Vajira & Francis Story)
The assumption that some of the Order were psychic enough to discern the particular jhana the Gautamid was in at the moment of his passing–that seems doubtful to me. The last assertion, that Gautama returned to the fourth of the material jhanas and was transcending that state at the moment of his demise, I think is the entire reason for the inclusion of this account.
In his early days, Gautama studied under two of the foremost teachers of his time. He attained “the sphere of nothingness” under one, and “neither-perception-nor-non-perception” under the other. He was unsatisfied with these attainments, and succeeded in arriving at “the cessation of perception and sensation” on his own. I have no doubt that this arrival was synonymous with his enlightenment, and that his insight into dependent causation and the four truths came from this experience.
“Cessation” is the cessation of action arising from determinate thought (AN III 415). What Gautama experienced was the falling away of habit and volition in perceiving and feeling, the cessation of habit and volition in the action of the mind (and what was left was “the disturbance” of the six sense fields, MN III 108-109).
Why did the elders posit that Gautama was transcending the fourth of the material jhanas when he died? The attainment of the fourth jhana is synonymous with the cessation of (habit or volition in) action of the body, and that cessation I believe was Gautama’s daily touchstone. In many of his lectures, he recounted the four material jhanas, and then spoke of “the survey-sign of the concentration”, leaving off the further states entirely.
I believe the fourth jhana was the primary cessation in his own practice of mindfulness, the fifteenth of the sixteen elements of his mindfulness. At one point in his teaching career, Gautama emphasized the sixteen elements of his own practice to his followers, as a thing “peaceful and choice, something perfect in itself, and a pleasant way of living too” (SN V 320-322). I think he understood that even though he taught “lack of desire” as the means to attain and transcend each of the jhanas, his followers focused their intent solely on attainment, to their own detriment.
That would be why the elders had him deceasing as he came out of the fourth of the material jhanas–to lay the emphasis on something more a part of daily practice, so the monks and nuns wouldn’t knock their brains out thinking they must arrive at “the cessation of perceiving and feeling” at the moment of their death.
The details of his teaching as far as I’ve been able to gather, are here.