From the "Annihilation" Event Book:
The Watcher may spend a doom die to create a "Timer" complication. This defaults to D6, but particularly time-crunched situations (as stated in the Action Scene) may start higher. Place the "Timer" in the action order as if it were a Watcher character. Whenever it acts, the complication automatically steps up to the next die size. If it reaches beyond D12, it automatically doubles and moves to the doom pool, giving the Watcher the power to end the scene. The "Timer" complication is also included in any Watcher reaction pool where time is a factor.
The heroes can target the "Timer" complication with tactics that could slow down the action. Players are encouraged to think creatively. Any such attack or action to reduce the "Timer" complication may be opposed to by a Watcher character in a position to oppose it; if nothing else, oppose the heroes with the doom pool plus the "Timer" complication.
If the hero's action total is higher, and the effect die is equal to or higher than the "Timer" complication, it steps back. Otherwise, it has no effect. As the Watcher, you may spend from the doom pool to step up the "Timer" if your reaction is equal to or greater than the "Timer's" current rating. Likewise, Watcher characters that are in a position to help can step the die up, unless successfully opposed by a hero.
The "Timer" can be used in cases where the action is so powerful that the complication can never be eliminated from the scene -- it can only be stepped back to a D4. The Watcher may move the complication back to the doom pool at any time., if the heroes prove to be too annoying or if they flee.