Force One's Hand

"This bet will force his hand, hehehe..."

Today's idiom is Force Someone's Hand, which means:
To force someone to act or speak prematurely or unwillingly. To bring about a situation which necessitates a person to act, often causing that person's plan to be revealed earlier than intended. To leave a person no other option but to act with whatever resources they have available to confront the challenge.

He didn't want to decide just then, but the board forced his hand.   
The daily oppression and injustice forced the people's hand.  They had no choice but to revolt.
That bet force's my hand.  I fold.  
If you don't tell me which one of you boy's broke the vase, you're going to force my hand: I'll have to punish you both.

 Synonyms: browbeat, hold a gun to one's head, impel

This expression alludes to the hand (the cards) held in a game such as whist or poker, in which a player is compelled to play some card from his hand or to reveal his hand.  This is done by increasing the bet,  "I see your hundred and raise you 200".  That player who is being raised has no choice but to match the bet or fold his cards (quit that hand, and thus lose immediately).