Not necessarily fast but highly coordinated. When most people throw punches or kicks, they're one at a time: left-right-left. When doing a flurry, you're employing techniques like Yama-tsuki where you combine a thrust punch to the face with a thrust punch to the abdomen in a single maneuver (most people will instinctively protect their face and take the shot to the abdomen). Jeet-kune-do or Wing-chun also come to mind. Watch them sparring with a wooden training dummy and see how they use hands, elbows, knees, and feet often simultaneously. I think this adequately demonstrates how Flurry of Blows is used in the actual choreography of the fight.
