Nikon D6 Reference Manual page 829

Remote Flash Control
Choose the remote flash control mode. Flash options can be
adjusted in the flash control display; the options available vary
with the option selected for [Remote flash control].
Option
m [Group flash]
[Quick wireless
x
control]
[Remote
u
repeating]
Radio Remote Flash Info
View the flash units currently controlled
via radio AWL.
Choose a separate flash control mode for each
group of remote flash units ( 0 670, 682).
Choose the balance between groups A and B
and adjust output for group C manually ( 0 673,
686).
The flash units fire repeatedly while the shutter
is open, producing a multiple-exposure effect
( 0 676, 690).
C The Photo Shooting Menu: Shooting Options
Description
777