noun
hidden ball trick
Baseball term: a trick play where a fielder hides the ball and tags a runner who thinks the ball is elsewhere.
隠し球は野球のトリックプレーの一つだ。
The hidden ball trick is one of baseball's trick plays.
noun
ace up one's sleeve; secret weapon
Figurative use: a hidden advantage, resource, or strategy kept in reserve for a decisive moment.
彼は交渉で隠し球を使うのがうまい。
He is good at using a secret weapon in negotiations.
あのチームにはまだ隠し球があるに違いない。
That team surely still has an ace up their sleeve.
切り札 is a trump card or ultimate resource, often the strongest single move. 隠し球 emphasizes the hidden or surprise nature of the advantage.
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.
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.