Official statement of cheating on PBW.

[Suicide Junkie]

ISTM that some sort of record of the game's data files are kept (encrypted) in the savegame file.

If your data files do not match, the game dosen't let you play your turn.
It does cause some trouble when you are trying to update the data files to fix bugs, or get a new patch of SE4; all the players have to make the change at the same time in order for it to work.