Brian Ratliff

shovelhd said it pretty succinctly.

First, you aren't working so much for someone, but you are working for the good of the team. When someone says they are working "for someone", it means they are doing things that will keep that rider well rested and therefore increase his odds of winning should the "predicted outcome" play out. IOW, if the predicted outcome is a sprint, and your sheltered rider is a sprinter, then you do things to save him energy. You tow him around the pack, you chase breaks that look like they might stick, you lead him out at the finish.

If there is more than one of you on the team working for a single rider, or if the predicted outcome is a breakaway, then you might cover breaks. Particularly the early breaks. The early breaks probably won't stay away, but in case they do, it is helpful to the team's goal of winning to have a presence in that breakaway.

When you cover a break, it means you are in the break but not doing any work to assist it. This has many implications. If the break stays away, then you are well rested and have the best shot at winning at the line. If a teammate can bridge, it means there are two of you, one of which is well rested, which dramatically increases the odds of winning if the break stays away. If the break is brought back, it means that your teammate didn't have to contribute to the chase; if your teammate is a sprinter, then he and his leadout riders didn't burn any matches worrying about the breakaway, if he is a breakaway artist, then he is well rested to counterattack with a break that might stay away.

Typically, when covering a break, you aren't really trying to actively slow the break down. All that happens then is it gets caught quicker and another breakaway, one which you aren't in a position to cover, goes up the road. You are merely being present in the break, getting a free ride to whatever end the break is destined.