To which timestamp exactly do the entries stored in the video_time_x files correspond?

To which timestamp exactly do the entries stored in the video_time_x files correspond? (light on the sensor, begin write, end write on hard drive?)

Good question, which I assume is highly related to another one: do the cameras share the same trigger signal? Are they fired at the same time or do they record in free-running mode and their streams are synchronized later based on timestamp?