To my mind, Allen, it's stability. Adding weight to the back end of the arrow is the same as removing weight from the front. It lowers FOC which decreases stability. Removing weight from the tail the same as adding weight to the front. It increases FOC and stability.

An arrow that's too weak in spine is unstable, just like one that's low in FOC. That's why I think adding weight to the tail of the arrow HAS THE SAME EFFECT as shooting an underspined arrow. It doesn't change the static spine. It doesn't change the dynamic spine. It simply MIMICS a spine change.

That's my thought process on it. Whether I'm right or not, your guess is as good as mine.