104.2. If a creature's power or toughness, a mana cost, a player's life total, or an amount of damage would be less than 0, it's treated as 0 for all purposes except changing that total. If anything needs to use a number that can't be determined, it uses 0 instead.
Example: If a 3/3 creature gets -5/-0, it deals 0 damage in combat. But to raise its power back to 1, you'd have to give it +3/+0 (3 minus 5 plus 3 is 1).
Example: If you control no permanents, the "highest converted mana cost among permanents you control" can't be determined, so 0 is used instead.
Ca veut dire qu'on considère que tu as 0 PV sauf si le total devait changer . Comme dans ce cas il change, ton total de points de vie sera de -10x2=-20 PV