My best guess is that it's the textfiles that bitcrack automaticly creates when you use the parameter --continue, those files contain information about the specific scanned range and how much of it that have been scanned since last update, which happend each minute
I supplied @ttdclient with such records my self.
I can generate such text file anytime for as much ranges as I want. There is no proof of work included in file.
Well, to a degree i know what you mean, but still the proof of work is there, as the file is written automaticly by bitcrack as scanning the specific range is done.
I guess one could tamper with the file, but then you would also tamper with the timestamps of the file it self, which would be a red flag that something is not what it's supposed to be.
Other than that, it's a matter of trust i guess
EDIT: reporting any range that you have not scanned will be very damaging for the hole pool project, as there would be a high risk for skipping a range where the key potentially could be located.