How does FileVault operate?

I launched FileVault yesterday morning (for the 1st time on a new startup drive). It's now 26 hours later and it's still only half done according to the on-screen run bar. If I terminate FileVault and then shut down (or force a shut down by holding down the on/off button), which triggers FileVault to rerun...will it start up from where I terminated it...or does it start over? I assume that it compares file states (eg modification dates) to the same file prior state and if the mod date changes FileVault re-encripts...otherwise it skips to the next file.