Canon Speedlite 600EX II-RT Manual (en)

Shooting with a slow shutter speed and second-curtain sync captures 
the trail of the light sources of a moving subject, such as car lights, in a 
natural way. The flash fires right before the exposure finishes (shutter 
Display <r>.
Press function button 4 <Y> to 
display <r>.
r Second-curtain SyncN
The second-curtain sync works well when the camera’s shooting mode is 
set to <bulb(B)> (bulb shooting).
When the flash mode is set to <a>, the flash fires twice. The first 
flash is a preflash to determine the flash output. It is not a malfunction.
Second-curtain sync is not available during wireless flash photography.
To return flash firing to normal, press function button 4 <Y> to turn 
off <r>.