I am translating a Chinese short film. In a dialogue, a soldier suggests to the king that they should activate a sacred time-travel item. The king knows full well that the time-travel item as their last resort. But he does not want to use it until he absolutely has to. So when he hears the soldier says, "we still have a trick up our sleeves -- Actus (the name of the item) ", the king is enraged and offended.
A related idiom that implies something important being held in reserve to be used only in the right moment is "Ace in the hole."
The term originates from poker, where players are dealt some cards face-up for all players to see, while others are face-down and only that player can view them- these are the "hole" cards. Since an Ace is a high card in poker, having a Ace in the hole is often an advantage. The hole cards are only turned over when all players have finished placing bets, thus waiting until the last possible moment to reveal the hidden advantage.
A secret weapon can convey the meaning you are suggesting:
- something or someone that no one knows about and that will give you an advantage over your competitors or enemies: Johann was the bank robbers' secret weapon - he knew how the security system worked.
I'd forward fail-safe. It is a way to defuse a bad situation.
A fail-safe or fail-secure device is one that, in the event of a specific type of failure, responds in a way that will cause no harm, or at least a minimum of harm, to other devices or to personnel.
- guaranteed to work; totally reliable.
- a fail-safe mechanism, system, or the like.
- unlikely to fail; foolproof
Of course, the use of this is less literal than the military definition.