(1) As Alan Brooks and John Kaufman and I have said in the past, every shaft will have the stiffest directions (that is, spines) at 180* intervals. Similarly with the most flexible directions (that is, NBP). If you measure anything else, there is something wrong with your measuring equipment. (Others have already noted that residual bend affects a spine finder's reading. That is probably the most common thing that is wrong with your measuring equipment.)
(2) FLO is important!!! It is not important because of anything the shaft may be doing during the swing (unlike a fishing rod), but it is one of the more reliable ways to find the REAL spine, untarnished by things like residual bend. In other words, FLO is a more reliable spine-finder than Colin's or Dan's. Slower perhaps, but it finds the real spine.
(3) There are three theories that I have seen about why spine alignment matters. NBP-COG is one of them. Here's the reasoning behind it:
* At the moment of impact, the major force bending the shaft is centrifugal force. (That is probably true, but not universally accepted. But let's proceed on the assumption that it is true.)
* That force will bend the shaft in the plane of the CG of the clubhead, because centrifugal force acts through the CG of the clubhead. (In essence, it is pulling the CG of the clubhead straight away from the hands.)
* If the shaft bends in a plane where the forces due to bending are not in the same plane as the bending, there will be spurious torque on the clubhead; you don't want that.
* But the only planes where the force and the bending are aligned are the NBP and the spine plane. In other planes, there will be some small angle between the bending and the force in the shaft. So you need to align one of those planes (either the NBP or the spine) with the CG of the clubhhead.
(4) If you build your clubs with nearly spineless shafts (like SK Fiber, or the new Harrisons, or many filament-wound shafts), then it makes little sense to say, "I used NBP-COG alignment [or any other alignment] and it worked GREAT!" You were aligning an effect that probably didn't matter one way or another.
Hope this helps, DaveT
