When I close my eyes and go to sleep, except for when I'm dreaming, I only ever know I've been asleep when I wake up. So each night as my conscious brain switches off, I experience a little preview of what death subjectively 'feels' like - the state of nothingness.
I know that for my method using an exit bag, first of all, I will experience either a blackout or a more gradual greyout and at this point, my conscious will still be active but functioning on a more primitive level. It's possible that I will experience a hallucination of life flashing before my eyes, re-live my favourite memories or intense emotions - perhaps I will experience the raw warmth and love I hold for a select few people in my life.
Eventually, I will begin to 'lose track' of those threads of thought and the actual moment of death itself will be more akin to 'forgetting to be alive', a lost thread of conscious thought that is simply never picked up again. I may be wrong of course, but that is the process as I understand it from my studies of neurology and consciousness. It may be a different process for different people or different methods.
I used to be terrified of death and that was a big reason for me not to CTB, but I honestly feel at peace now, and I'm actually calm about the process itself. If only the aftermath of my CTB was as reassuring and straightforward...