Yongnuo YN650EX-RF Manual page 22

Wireless flash shooting
(CH1/2/3/4) to choose.
Channel setting method: Short press the function button [CH] to
highlight the channel, turn the [dial] to adjust, and press [SET] to save
the setting.
The S1/S2 trigger mode is suitable for manual flash mode and TTL flash
mode respectively. When YN685EX-RF works in S1/S2 trigger mode, the
flash mode defaults to M mode. Short press the function button [+/-] to
highlight the flash output, and turn the [dial] to adjust.
S1 mode: When the flash is in S1 mode, it will trigger the flash
synchronously with the first flash of the master flash, the effect is the
same as using a wireless flash trigger. To use this mode normally, your
master flash must be set to manual flash. The TTL flash system with
pre-flash should not be used, and the red-eye reduction function with
multiple flashes also should not be used.
S2 mode: Also known as "pre-flash cancel mode", this mode is similar to
S1 mode, but it can ignore the pre-flash from TTL flash, so it can support
the master flash working in TTL mode. In particular, if the S1 mode
cannot synchronize with your built-in flash, you can try the S2 mode.
When using the S1 and S2 modes, the master control unit needs to turn
off the red-eye reduction function and the optical transmission wireless
flash mode, otherwise, the flash may be out of sync.
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