In several spiritual traditions, death is seen as a natural process, part of the notion of "impermanence" (the idea that all things are subject to change - except change itself, the only constant of life), and masters, known to be able to select the time of their physical death.
Instead of fighting it, when they feel their time in an incarnation has come, they enter a state of peaceful trance which allows them to gently drift to what they consider a transitory state, until their next incarnation or, when their soul journey is over, to the end of samsara (the cycle of reincarnations).