An ace up your sleeve
Definition 1 of 1
Pro Tip 1/3Subtly Revealing
Use the phrase when you reveal your advantage without showing it off or boasting about it. 
He smiled at the debate, knowing he had an ace up his sleeve.
Pro Tip 2/3Strategy and Timing
It's often about the strategic use of a resource or piece of information, revealed at a critical moment. 
Our new product is an ace up our sleeve for this fiscal quarter's earnings.
Pro Tip 3/3Unexpected Solution
The 'ace' should be something not obvious; it's a surprising solution that others aren't aware of. 
Faced with budget cuts, the manager had an ace up his sleeve: a private investor.