Nikon D6 Reference Manual page 224

3
Without releasing focus lock, recompose the
photograph and press the shutter-release button
the rest of the way down to shoot.
Do not change the distance between the camera and the
subject. If the distance to the subject changes, release the
lock and focus again at the new distance.
A
Taking Multiple Shots at the Same Focus Distance
•
If you locked focus by pressing the shutter-release button halfway, focus
will remain locked if you keep the shutter-release button pressed
halfway between shots.
•
If you locked focus by pressing the center of the sub-selector, focus will
remain locked between shots if you keep the center of the sub-selector
pressed.
172
Choosing How the Camera Focuses (Focus)