If a character falls below zero hit points, he is dead.
If a cleric can reach him, and pump curing into him fast enough - there is a chance that the soul of the chr has not yet departed for the final journey. It's like when the paramedics can get your heart re-started with that defibrillator.
Generally, your confused soul will stick around for d6 melees. You really don't know how long you have. The clerics tend to try to get you back to positive hit points until six rounds have gone.
When, you are positive, the GM will roll a d6. The resulting number is the melee your soul left your body and you are gone. If the cleric took 2 rounds to get you positive, and the Deathroll dice is a 5, you are alive. If it's a 2 - you are still alive (phew). But if it's a one - your soul is gone and you are dead. (Needing a resurrection spell).
Dragonrider's souls are attached to their dragons. They get a d8 for the Deathroll.