It is a blissful state of peaceful unconsciousness, a deep sleep from which you wake up forgetting all the past except what is bare essential. You will find a lot of material about it in the Mother’s writings. Here is what Sri Aurobindo describes in Savitri.
‘Immersed in voiceless internatal trance
The beings that once wore forms on earth sat there
In shining chambers of spiritual sleep .
Passed were the pillar-posts of birth and death,
Passed was their little scene of symbol deeds,
Passed were the heavens and hells of their long road;
They had returned into the world’s deep soul.
All now was gathered into pregnant rest:
Person and nature suffered a slumber change.
In trance they gathered back their bygone selves,
In a background memory’s foreseeing muse
Prophetic of new personality
Arranged the map of their coming destiny’s course:
Heirs of their past, their future’s discoverers,
Electors of their own self-chosen lot,
They waited for the adventure of new life….
All that is made and once again unmade,
The calm persistent vision of the One
Inevitably re-makes, it lives anew:
Forces and lives and beings and ideas
Are taken into the stillness for a while;
There they remould their purpose and their drift,
Recast their nature and re-form their shape.
Ever they change and changing ever grow,
And passing through a fruitful stage of death
And after long reconstituting sleep
Resume their place in the process of the Gods
Until their work in cosmic Time is done.
Here was the fashioning chamber of the worlds.
An interval was left twixt act and act,
Twixt birth and birth, twixt dream and waking dream,
A pause that gave new strength to do and be.
Affectionately,
Alok Da