NBA 2K10

A fastbreak is when your team gets the ball from a steal or defensive rebound and you move the ball to the other end of the court before the other team can get down. The easiest way to do this is to use two controllers. With the second controller, get the ball at the mid-court line. With your controller, go to the player with the ball and press to steal. Now get the ball and run down the court and get a lay up or a dunk.
You should get this just by going for the other achievements. The achievement unlocks right when you get the 20 points.