hidden ball trick
Baseball term: a trick play where a fielder hides the ball and tags a runner who thinks the ball is elsewhere.
The hidden ball trick is one of baseball's trick plays.
ace up one's sleeve; secret weapon
Figurative use: a hidden advantage, resource, or strategy kept in reserve for a decisive moment.
He is good at using a secret weapon in negotiations.
That team surely still has an ace up their sleeve.
奥の手 is a secret technique or last resort, often in martial arts or personal skill. 隠し球 is more about a hidden asset or strategy, not necessarily a personal technique.
Standard kanji spelling for both the baseball trick and the figurative secret weapon sense.
Alternate kanji spelling using 玉 (ball/jewel); less common than 隠し球 but still seen.
Compound of 隠し (hidden) + 球 (ball). The baseball sense is a direct calque of the English 'hidden ball trick'. The figurative sense extends the idea of something hidden and ball-like (a resource) to general strategy.