The Siddhas were concerned not only with prolonging life in the body, but transcending the experience of time altogether.
"Prana [the life-force] at all times will not die, O! My Dear!
If the vital air of the universe is infused and ascends the susumna [the central energy channel],
there is neither before nor after
In the union of the Yoga of absorption [Laya Yoga].
Love occurring will not leave; its excellence will be teeming (in) the vital air [in the prana itself], in sleeping, as well as in waking states."
- Boganathar (Verse 4, Astanga Yogam-24, in 'The Yoga of Siddha Boganathar', Babaji's Kriya Yoga and Publications)
"By performing pranayama, the vital air [life-force] enters the susumna leading to the Yoga of absorption.
Susumna stands for the principle of absolute non-duality and hence it is called the jnana nadi [the channel of supreme knowledge].
The highest stage in Yoga is reached when prana passes through this nadi.
After the prana reaches susumna, one enters into eternity where there is neither a before (a beginning) nor an after (an end), and there is no withdrawal or death afterwards.
Only for the multitude (who do not practice pranayama) is there birth and death, just as there is waking and sleeping."
From the commentary by T. N. Ganapathy: