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.

It would seem to me, that if you add weight to the rear, dynamic spine would weaken. The arrow would be heavier, adding more resistance on the shot. I would think that this would cause the arrow to bend less. Think of adding a pound weight to the rear. The arrow would travel very slowly and the needed spine to keep the arrow from bending, would be very light.

I completely agree with Arthur about the effects on FOC, which is why I never add weight to the rear. I don't dip my arrrows, use wraps, vanes, or any other heavier items on the rear. Randomly changing dynamic spine and lowering FOC is a double wammy in my opinion. The ideal arrow backend is light and has lots of drag, and adds good spin to the arrow.