On each turn where the character is unconscious (and has not yet been stabilized), they must roll their hit dice (as indicated by their class) to determine the deterioration of their hit points.
The result of the roll is subtracted from their total hit points, and this total is recorded as hit points below zero.
If the total HP below zero is strictly greater (and not equal) to twice the maximum value of the character's Hit Dice, he dies.
Death = HP below 0 > 2 x Max Value of Hit Dice
Example:
A cleric with 2d4 hit dice, is in a deteriorating hit points condition.
On his turn, if no other player has stabilized him, he must roll 2d4 and add this amount to the HP below zero.
If the total of HP below zero is greater than 16 (2 x 2d4), he dies.