The W ( N ) Button; Choosing A Flash Mode - Nikon D6 Reference Manual

The W ( N ) Button

During shooting, the W ( N ) button can be used to choose the
flash mode and adjust flash compensation for optional flash
units. Flash mode determines the effect produced by the flash,
flash compensation the flash level.

Choosing a Flash Mode

Hold the W ( N ) button and rotate the main command dial.
Option
[Fill flash] (front-
curtain sync)
[Red-eye
reduction] (red-
eye reduction)
[Slow sync] (slow
sync)
Description
This mode is recommended for most
situations.
The flash fires before the photograph is
taken, reducing "red-eye".
As for "fill flash", except that slow shutter
speeds are used to capture background
lighting at night or under low light.
The W ( N ) Button 131