The first time the command gets executed is immediately after maildirwatch starts.

Subsequent invocatons of command occur whenever there may be some kind of a change to the contents of the maildir.

maildirwatch does not determine what, if anything, changed in the maildir. maildirwatch might occasionally raise a false alarm and run the command even though nothing changed, but that's rare. It's also possible that by the time command actually runs, then whatever was changed in the maildir is no longer changed; it became what it was before it was changed (a mystery that will remain unsolved forever). It's up to the command to intelligently figure out if it needs to do something about whatever it finds in the maildir.