As your Avatar loses life, its connection with the realm weakens. When your life is reduced to 0, you don't instantly lose the game! Instead, you are at death's door. While at death's door, you can no longer heal or set your life to a higher number. The moment you reach 0 life, you can't die that turn. After that single turn, any damage dealt to your avatar is a death blow, causing you to lose the game. See the 'death blow' codex entry for more details. For example, on my turn, if I attack you and put you to death's door (e.g. 0 life), you cannot die for the rest of that turn. Then, on your turn and beyond, any damage to your avatar will be a death blow and I would win.