Hello,
Such watch would containt the filesize downloaded at the point when watch was set + when it was set. Purpose: after a month you could check specific watch and discover that for this file you didn't get anything in a month.
Currently it could be done as a workaround by manually renaming the file. It works, but such watch-marks would be more efficient, wouldn't conflict with filenames and would be user friendly.
have a nice day, bye