The clock-killing neg was a strategy employed during tournaments that use a clock. While leading by more than 5, but less than the maximum possible points in a tossup-bonus cycle, a team could buzz in with few seconds remaining, use the maximum amount of allotted time to begin a response, and then deliver as long an answer as possible, thus ensuring that time would expire before the trailing team could buzz. If a tossup-bonus cycle concluded with only a few seconds left, the team ahead could immediately buzz and just wait for time to run out.
Clock-killing negs were risky, since a player/coach in charge of keeping score might have added wrong and a team could actually lose the game by 5 points instead of win by 5 points. In addition, some moderators would cut a team off once it appeared that the team was attempting a clock-killing neg, allowing the trailing team a guess before time runs out.
A variant of the clock-killing neg could still used in formats with bounceback bonuses. After the trailing team has already missed the last question, if the leading team is up by less than (maximum possible bonus points - maximum possible tossup points), then it may buzz in and intentionally miss the question to prevent the possibility of losing on bonus bounceback points. In formats where the tossup goes dead once any team misses it, a team clinging to a slight lead with only one question remaining may also buzz in and say a wrong answer (or nothing) in order to preserve the victory. Many of those formats deduct the full value of the question for a wrong answer to prevent this exploit.
The 2008 change in the NAQT timing rule, requiring a tossup-bonus cycle to be completed if time expires after the tossup is begun has eliminated the clock-killing neg from pretty much all good quizbowl.