Looks like this is the original: Synchronisation
. Description says: metronomes (or "pendula") when on table, oscillate with random phases, since that is how they started and they are "uncoupled" (no energy/information flows from one to other so they do not "know" each other.) When they are all together on the cans, notice that the cans themselves oscillate little, providing coupling/information crossover. which forces "synchronization" in periodic systems (discovered by Huygens in 17th century).
A useful book: "Synchronization: A Universal Concept in Nonlinear Sciences " by Arkady Pikovsky, Michael Rosenblum and Jurgen Kurths.
A scientific article: http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=AJPI.
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