A void — having zero cards of a suit — means you can cut that suit whenever it's led. That's a free book, potentially. Short suits (one or two cards in a suit) are one turn away from becoming voids. Recognizing and creating them is a core JJDD skill.
Spotting short suits at the deal
Sort your hand by suit at the deal. Count each suit. A hand with 4-4-4-1 or 5-4-3-1 has a natural short suit — one card away from a void you can cut with.
The suit with a singleton is your best cut opportunity. Play the singleton at the first chance and you're void the next time that suit is led.
Creating voids on purpose
If you have two cards in a suit, dumping them both early creates a void by the third round. The book you're throwing them on is a book you were probably losing anyway — the payoff is the cut you can make later.
Discarding into books that other players are winning is the classic void-creation move. Somebody leads an Ace, you drop your 2 of that suit — you didn't affect the book, but you're one card closer to a void.
Weaponizing voids
Once you're void, don't announce it by cutting the very next book. Save it for a high-value book — an Ace being walked, a book your opponents need for their bid, a book your partner is about to lose.
Also: track opponents' voids. If they've discarded twice on hearts, they're void or near-void. Don't lead hearts through them — you're just handing them a free cut.
