Maelochs nailed it but there is one detail that can be confusing. Yes, there is a marked point everyone knows in advance where the race officially starts after say 5-19 km of easy rolling behind the lead car BUT: if the race official sees that someone has say flatted and is not yet back in the bunch at the start point, he waits until that rider is back while everyone rolls on some mor3e meters or even a kilometer or more before he waves the flag. Yes, that flag is for show but no, a rider cannot cross the imaginary line beh8ind the lead car until the flag is waved.

The point that is so confusing for folk familiar with almost every other form of racing is that time of race and distance of race don't really matter. Only what order and time differences riders cross the finish line. But a rider flatting in the neutral zone, then not being able to catch a field that heard he was back there and now was the opportunity to sink his hopes; now that 20 second wheel change could cost him an hour and a half on a long stage of a three week race. So race promoters give him a chance to get a decent start.

Now, once the race starts, fair play may well be off the table. If so-and-so big shot hangs at the back, one of the strong teams might get word and up the pace so much the field splits. A few years back, Alberto Contador got word that was the case as the field went into some early hills. Teamed up with some climbers and other strong riders from other teams to force an insane pace the whole race and sink the chances of more than a few. I believe he gave "goodies" (stage win, etc.) to others. He wanted the time. This one move sank the hopes of the Sky team in the days when they ruled the Tour.