What are the “cards” that Sydney holds? What is the “Ace” that Sydney has? (From Book 2 Chapters 6-10)

Sydney:

"I play my Ace, Denunciation of Mr. Barsad to the nearest Section Committee. Look over your hand, Mr. Barsad, and see what you have. Don't hurry."

Sydney has the power to denounce Barsard as an English spy.