Canon EOS REBEL T4I Instruction Manual page 29

Wireless Flash
Photography
You can use the built-in flash
for wireless flash shooting.
The camera's built-in flash can work as a master unit
with Canon Speedlites with a wireless slave feature and
wirelessly trigger the Speedlite(s) to fire.
Canceling the slave unit's auto power off
To cancel the slave unit's auto power off, press the
camera's <-X-> button. If you are using manual flash
firing, press the slave unit's test firing (PILOT) button to
cancel the auto power off.
Be Sure to also read the infermat!0n about wireless flash .........
photography in the Speediite's instruction rnanuaL