As I understand it Tibetan yoga merged with and was influenced by the Bon shamanic tradition.

I sometimes wonder if meditation was the tool used to reach these states are the tool used to manage these states.  For me it seems meditation works quite well for managing these states and releasing into them but I have yet to merge with the Godhead via meditation alone.  Though without a doubt my meditation practice has deepened significantly over the years. 

As for the "What if he dies quickly"

Well the Buddhists don't believe the brain is the mind.  To them there is no reason to not assume that he mind once separated from the body would be nothing but raw mind and this is were the sights and sounds of the bardo come from.  They do however put a lot of emphasis on the state of mind just before death... I equate this to mind set before taking a psych.  Good mind == good bardo.  Bad mind = Hell World.

Admittedly I'm still diving into Tibetan Buddhism so don't hold me as an expert on advanced theory and what not.

Peace.