Play your ace
Definition 1 of 1
Pro Tip 1/3Strategic Timing
Use the phrase when discussing a well-timed, critical action, often to gain advantage. 
In the chess championship, he waited patiently to play his ace.
Pro Tip 2/3Metaphoric Usage
Although derived from card games, the phrase is used in various life situations. 
She played her ace at the interview by demonstrating her unique skills.
Pro Tip 3/3Unexpected Solutions
Implies using a resource or tactic that others may not anticipate. 
Cornered by arguments, he played his ace: an overlooked legal precedent.