Reneging is playing a card you weren't allowed to play — usually failing to follow suit when you had a card of the led suit in your hand. In JJDD it's the cardinal sin, and the penalty is brutal on purpose.
What counts as a renege
The most common renege: a heart is led, you have a heart in your hand, but you play a spade (or another suit) instead. You're required to follow suit when you can. Playing anything else when you had the led suit is a renege.
It's a renege even if you didn't do it on purpose. Missed a card, wasn't paying attention, forgot you had it — doesn't matter. If the card was in your hand and the suit was called, you owed it.
How it gets called
A renege can be called at any point in the hand, up until the last card is played. The moment the last card of the hand hits the table, the renege can no longer be called — even if it happened four books ago.
Any player at the table can call it, but usually the opposing team catches it. Once called, everybody's cards get reviewed to confirm the renege actually happened. If it did, the penalty kicks in.
The three-book penalty
The renege costs the offending team three books, awarded to the opposing team. Those three books count toward the opponents' bid — often the difference between them making or breaking their contract.
In practice, a called renege is usually a hand-ending event. The offending team almost always gets set, the opposing team almost always makes. This is why paying attention to your hand and organizing your cards by suit before you play is not optional.
How to never renege
Sort your hand by suit at the deal. Look at what you have before the first card is played. When a suit is led, glance at the section of your hand for that suit before you play — a two-second habit that eliminates reneges entirely.
