Play one's trump card – use the advantage especially when others do not know about it

打出王牌 - 利用优势，尤其是别人不知道的优势

例句：

The CEO played his trump card by promising a dividend payout at the shareholders' meeting. (Card game)

首席执行官在股东大会上甩出杀手锏：承诺分红派息。（纸牌游戏）