Just read a magazine article whiich says that you can use this on some of Nikons digital Cameras but not on the D4 or D800.
By a cunning combination of AF-on,back button, then focusing with focus priority AF mode the camera will wait for your target to come into focus then trip the shutter. This was a useful addition on film cameras when taking wild life pictures,just focus the camera where you expected the animal to arrive and let the camera do the rest. Any one know of this or have tried it...Mike.