1. Initially there were two riders in the break working together I presume
2. The peleton is not going "all-out" all day long. There are periods when the pace of the main bunch may slow, or they have a moderate climb. This is when, very often a break solo or group goes off the front. The breakaway can last for a few K's or over 100K's depending on the day and the circumstances
3. If the individual rider or group of riders is no threat to the GC, then they are typically allowed to head up the road until . . .
a) That "on the road lead" becomes a threat to the rider/team in yellow
b) The key sprinter's teams( Telokom for Zabel etc . .) decide that they want to make it a sprint finish and catch the leaders to set up a final sprint finish for their key sprinters.
All that being said a long solo breakaway like that is a herculean effort and riders often struggle mightly to finsih the stage and in the days to come. But notice how that rider is spoken of, particularly if he is able to hold off the charging peleton until within sight of the finishline. He is often the "hero" of the stage, even though on the day he may have finished dead last on the day. Such is bike racing.
Steve Fleck @stevefleck | Blog