Drew,
You and Pedro may be communicating about this off list but to answer the question here, the tracking he refers to has to do with optical de-centration while zooming. In other words, the centers of the elements of each lens are not perfectly centered. This is usually small, and in 2D totally unnoticeable, but when you have two lenses trying to do the same exact thing, small errors can become larger. As you zoom from wide to tele, elements move around and any errors in absolute center are magnified. By adding the ability to recenter the lenses mechanically as Sean Phillips did with this rig or other Angenieux lenses allow with built in mechanisms, you can compensate for the de-centration and zoom through the focal range without introducing 3D misalignments.
