Panasonic Lumix S1H Operating Instructions Manual page 241

You can register functions to Fn buttons:
0
[ ] [
Setup] (l 367)
[Wireless FP]
The external flash performs FP firing (repeated high-speed firing of the
flash) during wireless recording, enabling recording using the Flash even
at fast shutter speeds.
[ ] [
Settings: [ON]/[OFF]
[Communication Light]
Set the strength of communication light.
[ ] [
Settings: [HIGH]/[STANDARD]/[LOW]
] [Fn Button Set]
] Select [Wireless FP]
] Select [Communication Light]
[Setting in REC mode]
9. Flash
[Wireless Flash
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