Nikon D6 Reference Manual page 206

Option
[3D-tracking]
154
Choosing How the Camera Focuses (Focus)
The camera tracks the subject using all 105 focus
points. If you keep the shutter-release button pressed
halfway after focusing on the subject in the selected
focus point, the camera will track the subject as it
moves through the frame and select new focus points
as required. 3D-tracking is available when AF-C is
selected for autofocus mode.
•
Use to quickly compose pictures with subjects that
are moving erratically from side to side (e.g., tennis
players).
•
If the subject leaves viewfinder, remove your finger
from the shutter-release button and recompose the
photograph with the subject in the selected focus
point.
Description