"When the arm of any metronome hits the side, it exerts a force on the blue platform. Normally friction would make that unnoticeable. But this platform is special. It’s set up on rollers so that it can move from side to side. When any two metronome arms hit, their forces on the platform either cancel out or add together, depending on how out of or in sync they are. Any arms that are out of sync will experience a force in the opposite direction that inches them closer to the pack."