We realize asking people to stare at a bunch of out-of-sync metronomes is kind of a hard sell, but this is actually really cool. For a while it seems like it's not going to happen, then, magically (note: technically physics, not magic), they do. Here's why.

To put it very simply -- because the platform they're on is suspended, it sort of bounces their movement back at them. This movement slowly effects all of them until they are all in sync with one another. For a much more thorough scientific explanation with much more unpleasant sounding metronomes that don't look as nice, look here.