Ace in the hole
秘藏的法宝(强大的备用力量)
a hidden advantage kept for later use that will help a person win (ace is a valuable playing card); alternatively, “an ace up your sleeve”
EXAMPLE:
“When he tried to get a job at the XYZ Corporation, John did quite well in the rigorous interview process, but he knew that his ace in the hole was the fact that his wife had a relative on the board of directors.