Panasonic LUMIX DC-GH6 Owner's Manual page 396

Video Settings – Synchronizing the Time Code with an External Device
•
When multiple units of this camera are synchronized, the time code and
exposure timings are synchronized so you can match the timings for starting
exposure between cameras.
•
The exposure timing can only be synchronized the first time after [TC
Synchronization] is set to [TC IN].
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