The metronomes in this video fall into the latter camp. Energy from the motion of one ticking metronome can affect the motion of every metronome around it, while the motion of every other metronome affects the motion of our original metronome right back. All this inter-metranome "communication" is facilitated by the board, which serves as an energetic intermediary between all the metronomes that rest upon its surface. The metronomes in this video (which are really just pendulums, or, if you want to get really technical, oscillators) are said to be "coupled."
The math and physics surrounding coupled oscillators are actually relevant to a variety of scientific phenomena, including the transfer of sound and thermal conductivity. For a much more detailed explanation of how this works, and how to try it for yourself, check out this excellent video by condensed matter physicist Adam Milcovich.
[ikeguchiLab]
1) Is why armies all over the world learn to walk both in lockstep, and counter step, for when they reach bridges, they must use the latter to avoid setting it to swaying to its destruction, a la 'Galloping Gertie' bridge in State of Washington; and,
2) Tesla used in his earthquake creating machine, which, (it is said) that he tore apart with a sledge hammer.(Don't know if that's true.)
R.C.