Usually you cover a break by being in it without working. Your team has an alternate plan and the break isn't part of it. Your responsibility, if you're covering the break, is to be there, be fresh, and do one of the following:
1. Act as a hopping step for a teammate's break out of the field.
2. Sit on, never pull, and be so discouraging that the rest of the break sits up too.
3. Sit on, pull sometimes, and wait to get caught. You're forcing lots of guys to work to catch the break and your team will be able to sit on.
4. Sit on, never pull, and when the rest of the break drags you to the finish, win the race.
#4 is controversial because you didn't contribute. But if you say "I can't work, my guy is in the field, AND I WILL SPRINT AT THE FINISH!" the the break doesn't have much to say. You'll be called names for a while though. However, if they drag you to the finish and you've told them you're going to sprint, their bad, not yours. If you tell them you're not going to sprint, you better not.
Sometimes the act of covering the break involves catching up to it. In the Tour du Pont tape, some Danish guy takes off. A few seconds later PDM launches a guy, so does Z, and they both bridge about a km later. Then they looked at the guys in the break like, "Yeah, so what?" and everyone sat up.
cdr