so to answer Justin: that is the terminology of the lesson/video course M121 on the aggregation pipeline. we are taught the short mode for command line i.e.:
{$unwind: “$genres”},
at the end of the video the long mode is noted/introduced but only that FYI it exists - and nothing further (the 3 parameters per Prasad’s reply)
I am experimenting with the course assignments in doing them both as shell commands, and then over in Compass. The class is pretty much all shell commands.
Over in Compass, in the Aggregation tab, when you select stage $unwind - - it presents the long mode…so that was a head scratcher… but Prasad has answered it in providing the syntax to be:
path: “$arr” i.e. path: <$field name>
…which maybe was so easy, in retrospect, that it never occurred to me…