I found a bit of folklore on that https://utcc.utoronto.ca/~cks/space/blog/unix/TheLegendOfSyn...

Bastard operator, checking in. Cut my teeth on paper, then tape, then disk.

    $ sync;sync;sync # three syncs in rapid succession is interpreted by the tape machine as a rewind command .. on card tray, shuffles cards to settle

    $ sync # one sync, just syncs to tape at current stream position

we always used sync;sync;sync followed by a reboot command