I agree that MBTI does a poor job explaining itself. The only concepts from MBTI that I even use are the four-letter type abbreviations and the functional orders--which are NOT the only functional orders that exist in practice, but rather represent ideal personality balances which provide the most utility.
To me, the functions do not represent individual specific actions, but rather broader value systems which govern the most basic motivations for everything we believe. I am in the minority on this forum because I do not believe that shadow functions are really used at all, but only appear to be used when a similar function reaches a similar conclusion.
I believe that all four functions combine to produce one fluid value system with varying levels of influence from each function. Again it's crucial to recognize that the functions are not single actions but rather complete value systems based on the way in which the world is perceived and judged. In this context, a person cannot use Fi and Ti because these value system contradict each other regarding how internal judgments should be made. Whenever a person uses Ji for anything, if you pry enough you can always discover whether Fi values or Ti values were the root cause--regardless of whether they happen to agree on the surface.
Here is an example of my explanation of shadow functions:
I may "use Fi" sometimes, but not because I place any fundamental value in Fi itself, but rather because I recognize situations where Fi's values happen to align with my own (which are invariably the result of Ne+Ti+Fe+Si.) I have no shame in admitting that I find Ti a totally superior system for internal judgments, but then--of course I do, I'm a Ti user! Again you need to direct your focus toward the total reasoning process and its most basic underlying values, not just the surface behavior or end conclusion.