When you put the unit in Trac Lock, it will lock the GB to the current setting the machine calculated when you ground balanced in the Ground or Salt setting. When you are in Trac Lock, you can then use the Ground Grab button to update the GB if need be. Ground Grab is an easy way to get the most current GB conditions, instead of going back into Ground or Salt, re-grounding, and then going into Trac Lock again.
If I am incorrect on this, someone please correct me. I just got an MXT Pro a few weeks ago, and this is how I interpret what I read in the manual.