Canon Speedlite 600EX II-RT Manual (en)

When using an EOS DIGITAL camera released in and after 2012, you 
can perform remote release (remote control shooting) from a 600EX II-
RT set as a slave unit during radio transmission wireless shooting.
Display <N>.
Press the slave unit’s function button 
4 to display <N>.
Take the picture.
Press the slave unit’s function button 
1 <U>.
A release signal is sent from the slave 
unit to the master unit, and the picture 
is taken.
Remote Release from a Slave Unit
Remote release is not possible from a slave unit with cameras released 
up to 2011 or with EOS REBEL T6/1300D or EOS REBEL T5/1200D.
Shooting is not possible when autofocus fails. It is recommended that 
you set the focus mode switch of the lens to <MF>, manually focus on 
the subject, then release.
Remote release is performed with “Single shooting” regardless of the 
camera’s drive mode setting.
When there are two or more master units (p.72), the release signal is 
sent to the main master that has the <D> lamp lit in green.