(someone's) ace in the hole

Translations
чей-то козырь в рукаве
She kept her knowledge of the new software as her ace in the hole during the job interview.
Она оставила свои знания о новом программном обеспечении как козырь в рукаве на собеседовании.
Definitions
A hidden advantage or resource kept in reserve until needed, especially one that gives someone a decisive edge in a difficult situation.
Words
ace

noun
туз
hole

noun
дыра