Iron Will is the upgrade to Recover Health that saves Jin from a killing blow. When you go down you can spend Resolve to get back up with one point of health.
On a mortal wound, trigger it to recover instead of dying. You are left with a single point of health, so get out of danger at once.
Keep enough Resolve banked for your difficulty, or the save won't fire.
Archery
Aim and fire a bow.
Assassination
Kill an unaware enemy with a quiet stealth attack from behind or above.
Chain Assassination
Assassinate two nearby unaware enemies one after the other.
Chain Assassination Master
Chain up to three assassinations in quick succession.