What triggers folder action scripts?

I recently set up the DTPO folder action script for Import, OCR, Delete, as described in an older DT blog entry. I know it was working yesterday, as at some point a log appeared in DT displaying progress info from ABBY. There were only 2 files in the folder, but they were quite large (academic monographs). But then my MBP went to sleep (and I guess “power nap” doesn’t include folder action scripts) so when I woke it up this morning everything was just as if the script hadn’t run at all. I figure ABBY was still in the middle of OCR-ing when the machine went to sleep, so I need to start the script over again, but I don’t understand how that works. What is it that triggers a folder action script, making it go to work?

The scripts shipping with DEVONthink Pro are triggered after adding items to the folder.