It is a different type of race called Keirin. Both men and women race and they are brought up to pace behind a motorbike, then they sprint from full speed till the finish.
The olympic commitee need to scrap the omnium and bring this event back for Rio. This is one of cycling's blue riband events and shouldn't have been dropped in the first place. The paralympics 2012 showed that it's exciting and it would be brilliant to watch Wiggo defend his gold medal from Beijing.
With the motorcycle bringing them up to speed it stops the bullshit you get in the individual sprint of them not going or even stopping in order to get position. Everyone gets a slipstream so it promotes a very fast race.