@horqua@mastodon.social @wonderofscience@mastodon.social This is my educated guess: It's dependent on the metronomes being imbalanced. Normally, a metronome is on a stable surface that it can brace itself against. The small lateral forces caused by the imbalance will then be taken up by friction. When that surface is easily movable, friction is still high to the immediate surface, but that surface in turn will start to move laterally in sync with the metronome. This enables the energy transfer that @minzemitzimt@nerdculture.de mentions.