Very briefly:
1) Activate Mark of Death. A timer starts.
2) Target receives X amount of damage from other attacks, from yourself or other party members. The value X is called the stored damage.
3) Before timer expires, toggle Mark of Death. Target receives f(X) damage, where f() is the bonus function.
Not sure if the bonus function is additive, like X + Y, or multiplicative, like X * Y.
This is why MoD is so OP, because it stacks damage. A common "quick kill" trick is to have a Tempest rogue as a companion. Activate MoD, have the Tempest do Thousand Cuts on the target, and when that is done, trigger MoD. Double TC damage plus bonus damage! There are some good videos of people finding various ways to stack max damage, like static cage + DoT + MoD, or multiple Decloaking Blasts + MoD, etc.
UPDATE:
From the community guide:
http://forum.bioware...ombat-mechanics
Mark of Death
- Deals 100% of stored damage when manually triggered. Deals 50% of stored damage when allowed to expire. This damage, although displayed as physical damage in-game, ignores armor, all resistances, and is not affected by attack_bonus, damage_multiplier, flanking_bonus, or critical_damage_bonus (see below).
- [Bug? - Patch 2] Although the damage from this ability can trigger critical hits, they do not receive any critical damage bonus. Effects that proc on critical hits will still be triggered, though.
- [Bug - Patch 2] Sometimes deals no damage when allowed to expire.
So I guess what the Codex entry means by "bonus damage" is that it does not take half the stored damage away.
f(x) = x * 1, instead of x * 0.50.