Think there is some uncertainty in my mind about the difference between tracking and locking on and locking focus point and locking focus.... very different expectations of what the Af module does. The words are similar but trying to read thru various posts I get the feeling that the writers are a little loose with terminology. Maybe I should read the manual, but here goes.... Chime in with corrections please!
Tracking means following a subject. The camera offers two dimensions of tracking beyond what the guy holding the camera does: tracking in the X-Y plane to keep the active sensors on the intended subject and tracking to predict how far or close the subject will be at the instant of actual exposure ( versus at the time when the shutter is fully pressed). You get the latter whenever in Af-C mode. And I'm guessing this depends on the camera calculating the subject velocity in the Z axis ( coming closer or farther) and knowing shutter (and lens?) response times to get the lens to the right place for when the sensor actually sees the subject. More on X-Y sensor tracking later.
Focus locking is simply having the camera make a final calculation/decision on where to focus the lens and that's it. No more fiddling. Signal is sent to lens when you half press in AF-S...... or last signal sent to lens is retained when you take thumb off the AF-On button in AF-C mode. You can reframe/move the active sensor off the target and the camera doesn't care or do anything 'cause it already did its job.
Locking the focus point is a electro-mechanical thumb command that you make manually to choose (and lock down) which sensor initially will be active for establishing the subject on which you want the camera to focus.
Now "locking on" sounds to me like something more related to X-Y tracking.... i.e. something to do with letting the camera follow a subject after initially identifying it as a focus target. Only Nikon knows how the Af module does it, but what it does (my interpretation!!!) is to make an initial distance measurement with the selected focus point ( you picked it with the manual focus selector and lock button and ring) and then (only in Dynamic area mode and AF-C mode) it assigns a whole bunch of sensors ( you pick from 9 to 51) the task of continuously recalculating distance to THAT subject no matter where it may wander in the viewfinder. It's like having one FBI agent assigned the inital task of finding the spy in a crowd in front of the train station, but then alerting a whole team to keep watch for the spy's movement in the crowd. The 3D feature in the 51 point selection even lets the camera make its evaluation in three separate color channels. The color signature of the subject therefore aids in tracking. How it knows THAT subject has moved and reappears within the view of another sensor within the stake-out group is a mystery to me.
Oh yes, a4 Focusing Tracking with Lock On .... that is a command to the FBI agents to wait just so long ( Long, Normal, Short or not at all) to acquire a new subject (i.e. go back to the agent responsible for inital subject acquisition) in the event that the subject momentarily disappears behind trees or something and no sensors can see it. If you set it for Long it means you can't tell the agents to quickly re-focus on a new subject once you've lost the one you've been tracking. But it also means that if you don't wait long enough and are too twitchy, you'll be shouting at the acquisition agent and he may find you unintended suspects in the crowd.... or even a statue in the middle of the square.
So that's how I see focus tracking operating and the AF-On button's function is to initiate the tracking and suspend tracking.
Now that I've revealed my ignorance, sensei, please help me understand!
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Marabou Muddler
http://MMuddler.smugmug.com/Sports 
