Nikon D6 Reference Manual page 684

6
Take pictures.
•
Pressing the shutter-release button on the master camera
releases the shutters on the remote cameras.
•
Note that the standby timers will not expire automatically
when the cameras are in synchronized release mode.
A
Remote Camera Check
If [On] is selected for [Remote camera check], warnings will be displayed
if any of the remote cameras are not ready.
632
Synchronized Release