How does virbedit sync fit and mp4 files?

I'm undertaking a research project involving thousands of hours of VIRB footage that I'm using a script to cut up video based on the fit data.

I can't figure out how virb syncs the video and fit data.

Is the data synced by just assuming the start time of the file is the same, or does it use a particular bit of metadata inside the mp4 file?

Thanks!