The third day of track cycling in the Olympic velodrome means the start of the keirin.

The what?

Only the strangest, most interesting event in the program, an event that began as in the 1940s in Japan as a gambling game and that joined the Olympic program at the 2000 Sydney Games.

Here’s how it works:

There are six riders in each race, and they draw lots to determine their starting position. Then, they begin riding laps _ there are eight total on the 250-meter track _ behind a motorized bike called a derny. The derny gradually picks up speed as the laps tick off, starting at about 15 mph and finally reaching about 30 mph before exiting the track a little over two laps from the finish.

AP Photo/Alastair Grant

At that point, it’s all up to the cyclists. They begin bumping, pushing and sprinting with the first to cross the line first. It’s a tactical game as much as a physical one, though. Position matters with the riders second and third in line benefiting from the draft off the leading rider.

“The keirin is a crapshoot,” American rider Matt Baranoski explained. “It’s just a matter of who has a little bit of luck out there.”

Just imagine a two-wheeled NASCAR race at the tight oval of Bristol Motor Speedway.

What is the point of the derny? Well, it's two-fold: It gets the crowd into it as the race picks up speed, no small thing when the sport's roots are in betting, and it gets the riders a slight bit worn out. When you are sprinting at more than 45 mph at the finish, even the energy expended those first seven laps can make the difference between failure and winning a medal.

AP Photo/Alastair Grant

Sure, luck plays a big part of it. The drawing of lots and starting order gives some riders an edge over others. But there is also skill involved. That’s why certain riders, such as Britain’s Chris Hoy, have had sustained success in the event. He won the last two Olympic gold medals before retiring shortly after the 2012 London Games.

One thing is certain: The raucous crowd inside the velodrome loves it.

“It’s a derivation of a betting sport in Japan, so you have to get the crowd into it,” Baranoski said. “It’s a battle. You just need to have the right mentality going into it.”

The women’s competition is Saturday while the men compete Tuesday.