Canon EOS 750D Instruction Manual page 180

Using Wireless FlashK
*1: If the Speedlite does not have a transmission channel setting function, it
operates regardless of the channel set on the camera.
*2: In small rooms, the slave unit may work even if its wireless sensor does not face
the camera. The camera's wireless signals can bounce off the walls and be
received by the slave unit. When using an EX-series Speedlite with fixed light-
emitting unit (flash head) and wireless sensor, take pictures while making sure it
can fire.
Canceling the slave unit's auto power off
To cancel the slave unit's auto power off, press the camera's <A>
button. If you are using manual flash firing, press the slave unit's test
firing (PILOT) button to cancel the auto power off.
The camera's master function cannot be used for wireless flash shooting
with radio transmission.
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