empty quotes for rsync parameter

[Wolfram Volpi]

Why does inserting an empty quote ("") into a rsync command cause the current directory to be backed up?

Not sure if this is a UNIX-syntax question or rsync-syntax question.

Code:
#!/bin/sh

echo "\nspace between quotes has no effect; only music is backed up:"
rsync -ilrtn " " /home/wolf/music /backups/backup1

echo "\nempty quotes backs up current directory and music:"
rsync -ilrtn "" /home/wolf/music /backups/backup1

Thank you.