I think in my reply above I was confused. It looks like I was talking about using Select TOCHECKS to take the selected files and flag them for synchronizing.
Right now, at least, Select FROMCHECKS doesn't seem to do anything, as you say. (Maybe it worked when I wrote my reply, but the way I described things make it sound like I was using TOCHECKS and not FROMCHECKS by mistake.)
If you're not doing this recursively -- that is all the files you're interested in are at the top level and not in any subdirectories -- and you just want to know which files are missing between two folders (or which files are the same) based on their names, then the Select SOURCETODEST and Select DESTTOSOURCE commands can be used for that.
(See also here, here and here for some threads about using those commands, sometimes in complex ways.)