Canon EOS Rebel T6i W Instruction Manual page 184

Easy Wireless Flash ShootingK
Fully Automatic Shooting with Multiple External Speedlites
You can have multiple slave units fire as if they were a single Speedlite.
This is convenient when you need a large flash output.
Flash Exposure Compensation
If the flash exposure looks too dark or too bright, you can set flash
exposure compensation to adjust the slave units' flash output.
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Basic settings:
Flash mode
E-TTL II meter. : Evaluative
Built-in flash
Channel
All the slave units will fire at the same
output and be controlled to obtain a
standard exposure.
No matter which firing group (A, B, or C)
the slave units belong to, they will all fire
as one group.
Select [1exp. comp.], then press

<0>.
If the flash exposure is too dark, press the

<Z> key to increase the flash exposure
and make it brighter. If the flash exposure is
too bright, press the <Y> key to decrease
the flash exposure and make it darker.
: E-TTL II
: EasyWireless
: (Same as slave units)