This example is based off a play that one of my opponents made at a recent match. Suppose that your team is winning 499-341. You are one point away from winning, but also one bag away from going back 100 points.
The bids are:
Your partner: Nil
Opponent to your left: 5
Opponent to your right: ??
Regardless of what cards you have, you should bid ... 9! You're almost certain to be set, of course, but with a successful nil your team will net 10 points for the hand and win without the risk of getting any bags. If you have a competent partner, she would only nil in this situation if it was pretty safe, given that she's first to act and that your team is well ahead. You can now focus on covering the nil without having to worry about how many tricks you take.