In my experience, bare shaft tuning indirectly establishes the correct FOC point for a given bow, shooting style and shaft spine.
If you're shooting quality carbons, the bare shaft process will eatablish the best FOC point for that make and size shaft. By best, I'm talking best or most foregiving arrow flight.
When you find this point, the bare shaft will fly as straight as your fletched arrow. If you move the FOC foreward or behind this point, your bare shaft will not fly through the air with the nock in line with the point.
If your bare shaft is not flying as stright as your fletched arrow and field point, chances are your broadheads will fish tail.