This shouldn't be difficult to accept. There's a reason it's in the 1D X, and purported to be in the 7D II as well. But if you want to see a great implementation of it - just try a D810. Seriously, just try it. Go to a camera store, and slap some 24/1.4 primes on some bodies. Put a 5D3 in 'auto 65 point selection' mode with AI servo, a 1D X in the same mode but w/ iTR engaged, and a D810 with 3D AF tracking mode engaged. Start on a subject in the center (or whatever focus point you've chosen), then move the camera around wildly (or have the subject dance/run around if you want/can). Be amazed at what how the camera can swiftly move the AF point to stay on the subject. Or not.
Are you saying you can slap on a fast prime, focus dead center, and then rotate ("recompose" generally involves much more rotation than translation, obviously) the camera about its own axis so the initial subject as seen by the AF unit goes from one side of the frame to the other (or at least from the borders of the PDAF sensors) and significantly out of the DOF and it will track it?
If so, that's cool and I do not believe my 5D3 could do such.