The point of having the correct spine and a tuned bow and a tuned arrow is to get the maximum amount of stored energy in the bow transferred to the arrow and then for that energy (transferred energynow in the arrow)to be effective on the target. If the bow is not tuned it will not transfer enrgy efficiently. If the arrow is improperly spined/not tunedit will not accept energy from the bow efficiently and it will not be able to use what energy it has effectively on the target.

This is an extreme example but... If the arrow hits the target sideways (extremely bad setup)you will not get any penetration. The straighter it hits the target (less major oscillations) the better the penetration. When you have a correctly spined/tuned arrow shot from a correctly tuned bow you should get maximum penetration. And that is what we all want maximum target penetration/passthru's.