Arrow Spine. Another "other thing" to be addressed.
"Spine" is one of those terms which means different things in different context.
STATIC SPINE is the stiffness of the arrow and its resistance to bending. To determine statice spine, the arrow is supported a two points a known distance apart and a force is applied to the center of the distance, usually through a weight. The amount of displacement of the center point, how far it drops, determines the spine. Arrows with a high spine will not sag as much as arrows with a low spine.
The factor which determines the spine is the stiffness of the materials in the shaft and the geometry of the shaft. In multi-layered arrows (carbon, aluminum, etc.) the bonding materials also contribute. The inside diameter, the cross-section shape, the thickness of the material all contributes to the Static Spine.
The Spine never changes unless the arrow is broken somehow.
DYNAMIC SPINE is how much the arrow does bend when fired. This depends upon stiffness (Static Spine), string force, fletching, nock weights, etc.) "Weak" and "Stiffness" are often used in terms of Dynamic Spine.
It is more difficult to determine Dynamic Spine than Static Spine because all the simple formulas don't work with such a massive force acting down the shaft. Consulting a Machinery's Handbook or other Engineering manual will give beam formulas to determine Static Spine, but when dealing with a large compressive force, "all bets are off."
The third meaning of "spine" is that of arrow direction. If you replace the arrow with one that bends more, the arrow will tend to go to the right when shot, at least for a right-handed archer.