Anything that is born is sure to die at some point in time. In fact, the moment it is born, it has already begun to die like the unwinding of a clock. It is only a question of time. In the same way, the moment of our death should be the moment of our birth also.
What is Death really?
How does it happen?
When does it happen?
Why does it happen?
What dies really?
Where does it happen?
Who is the cause?
These questions often stare at us, generally with no answers except to take refuge in the great scriptures and the experiences of great saints.
They say that the human being is a complex of body, mind/consciousness, and soul or the spirit or the being. One is subtler than the other and the soul or being is the subtlest entity. And it is the body that dies NOT the other two - mind and soul. In Nature, anything that is subject to change dies.
The human being is said to be comprised of three bodies - Sthool Sharir (Physical body), Sukshma Sharir (Subtle body), and Karan Sharir (the Causal body). And what we call Death is nothing but the death of Shool Sharir.
The human being is so structured that they say it comprises of 5 sheaths/layers/koshas - Annamaya kosha (the layer of Physical body), Pranamaya Kosha (The Life Force layer), Manomaya Kosha (The layer on Consciousness or Mind), Vignanamaya Kosha (The layer of Wisdom or real knowledge) and Anandamaya Kosha (The layer of Bliss). All the koshas are spread throughout the human frame, they are so intermingled that they cannot be separated. One layer s subtler than the other. At the time of death, all these layers withdraw one by one. So our Saints say, that Death happens by stages and not suddenly.
When a human being dies, they say, "he is gone." In every language strangely we speak the same thing "Mar gaya" (Hindi); Chanipoyaadu" (Telugu); "Satthi poyitaan" (Tamil); and so on. That is as long as the human being is alive, we refer to his body but when he dies we refer to something other than the body. Though the body is very much there in front of us. But as long as we are alive each one of us identifies ourselves with this body. The body-consciousness is predominant. Therefore the fear of Death.
Nobody precisely knows to this date, as to when, how, why, and where the death happens. That is why all have to agree that it is in the hands of the Creator (if any) or the Unknown called God.
How to know what is Death? There is only one way perhaps. And that is - one has to experience Death before the actual Death as one of the saints said. Is that possible? Yoga says, Yes. Especially the Heartfulness Sahaj Marg System says, Yes. By establishing ourselves in the soul-consciousness thoroughly detaching totally ourselves from other layers of Consciousness, perhaps that is the ONLY possibility.
This System of Yoga teaches us how to die every day in an incremental fashion and not being aware of it. (But not to worry, every time there is death, there is also birth. So while learning how to die properly, we also learn how to live properly through this Yoga. It is both an art of living and the art of dying.)
During Death, the physical organs, the senses, the life-force, (the prana) everything is withdrawn stage by stage. Something like one achieves through Samadhi condition. In Yoga, we consciously practice this withdrawal with the help of the Guru or Master and experience Death while the body is still alive. Perhaps it can be called a sort of spiritual simulation of death. And when one experiences Death in such a way, it is said that one achieves mastery over death and life and perhaps the choice or need for being born again or to die again rests with the Soul. And the spiritual journey or Yatra continues towards Infinity.
