This probably has to start from the time when the Gate of Life opened automatically.
The Gate of Life, among The Eight Gates of Heaven, according to the recorded texts in memory, once reached the stage of Mahayana, could bring dead back to life and animate dry bones; it could also improve one's physical body, enough to achieve eternal life.
Of course, that was absolutely impossible now; you could only slowly absorb the surrounding spiritual energy by opening the Gate of Life, and once it closed, you were isolated from this energy.
While absorbing spiritual energy, you found that although it entered your body like a stone sinking into the sea, hard to trace, it brought a wonderfully strange sensation to your body. This feeling was profoundly mysterious, impossible to describe in words; you just knew it was beneficial for your body and cultivation, so you persevered tirelessly.