ZBB = Zero Bug Bounce
As we start moving from the writing code phase into the fixing bugs phase, the development teams start pushing towards zero bug bounce. If you've noticed that the number of blog posts from the C# team has down a bit recently, that's one explanation.
Why is it called zero bug bounce?
Well, in the days and hours coming up to the ZBB date, teams will try really hard to get all their active bugs resolved, so they can declare, at one point in time, that they reached ZBB (they should technically say that they reached ZB, not ZBB, but thats not the way it works).
Reaching ZBB demonstrates two things. First, it shows that you have gotten rid of your bug backlog, and it also shows that your fix rate is greater than your incoming rate