The event is handicapped race, where riders are graded on ability and set off in separate groups, with the slowest starting first and the fastest starting last (the "scratch markers").

the intent of the handicap is to give everyone a fair chance of winning. If the handicaps are right then everyone is together in the final few kilometres. 

The format makes for some great action, with every group working hard to stay ahead of those groups chasing them and/or trying to catch those in front.