Canon Speedlite 600EX II-RT Manual (en)

 Manual Flash/Stroboscopic Flash Setting on a Slave Unit
Set the stroboscopic flash settings. For 
details on the setting procedure, see 
page 42.
Stroboscopic Flash
The flash frequency when performing stroboscopic flash during optical 
transmission wireless shooting can be set from 1 Hz to 199 Hz (settings 
from 250 Hz to 500 Hz are not available).
A slave unit that is set as an individual slave cannot receive flash mode 
control from the master unit. The slave unit fires in the flash mode that is set 
by individual slave.