Most people should be familiar with the old phrase "Ace In The Hole". It refers to something very special only to be used when things are not going as planned or at the most appropriate time and place to insure success.
In football, a coach may have his team practice a certain play only to use that play once in a game or an entire season. Then again he might not use that play if he does not have to. That play becomes his insurance in case things do not go as planned and his team is in trouble. That play thus becomes that team's "Ace In The Hole".