Roller Derby 101
That's a good video up there, but for those who need to read, here's a basic look at how flat track roller derby is played.
A bout is 60 minutes long, divided into two, 30-minute halves. Each "play" is called a jam, and is timed to a maximum of two minutes.
A standard jam includes five skaters from each team, lined up in a 1-3-1 formation. One skater from each team sets the pace from the front, with the middle six skaters (three from each team) making up the pack. The final skater from each team, called the "jammer," starts several seconds after the rest of the pack.
The jammer's goal is to get through the pack without skating out of bounds. Skaters in the pack have two goals: (1) get their jammer through, and (2) stop the other team's jammer from getting through. This is where the jostling, hip checks, booty blocks and other
nastiness fun happens.
Once the jammer gets through the pack, she must skate around the track, and then scores points for her team by skating cleanly past skaters from the other team (by cleanly, I mean staying in-bounds). Thus, if she makes it through the pack a second time, she'll have past four members of the other team, scoring four points. Any skater, including the jammer, knocked out of bounds, must come in-bounds behind the skater who knocked her out.
The first jammer through the pack has the ability to end a jam before two minutes have passed. She might do this so that the other team is unable to score.
There are, of course, penalties and strategies, but them's the basics, and if you're watching a bout, that's what you're seeing.