I used something like that in my last campaign.
The mechanism was simple : each player has one "fate point". The GM also has one.
A fate point can be used to prevent the death of a character or NPC on the scene of the action. It can be used for anyone, the player's character, another PC, an allied NPC, a bystander, or even an enemy. The character or NPC will end up alive later, and the players and the GM must work out how this is possible. While "presumed dead", the character or NPC has script immunity (they cannot be "poked just to make sure").
Once a player has spent their fate point, it goes into the GM's hand. Once the GM spends a fate point, it goes into the hand of a player, preferably either the one who was most involved in the "death" of the character that was just saved, or one who currently has no fate point if any (it's a guideline only).
And that's all.
Over a five year long campaign, I think three points were used - one by me (on an NPC), two by players (one on their own character, the other on an NPC).