Spine is deflection. Deflection is how much the arrow bends when supported on the pegs (these can be either 26" apart (uisnfg a 2lb. weight) or 28" (using a 1.94lb. weight) depending on whose measurement you are using) and is measured in thousandths of an inch.
For instance, a shaft measured over a 26" span using a 2lb. weight that "bends" or deflects .520" is said to have a 50lb. spine. It doesn't matter what the material is, that's the spine rating.
There are 2 different types of spine. Staic spine. Which is described above. And "dynamic" spine. Which is how the arrow actually behaves when we shoot it from a bow.
Different materials behave differently "dynamically". Also, different archers have different "dynamic' affects on arrow spine. As does the amount or lack of centershot in the bow set up. The type of string material being used. The mass of the string silencers being used. The way the archer release the string, etc..
Every 1/16" difference in centershot has an afect of increasing, or decreasing the spine reguirement of a bow.
For example. A bow that is 50lbs. at 28" and hhas a sight window "cut to center" will likely require a 50lb. spine for a 28" BOP arrow. However, the same bow, with a sight window that is 1/8" past center, will require an arrow that is 10lbs. heavier in spine. Tremeber the 1/16" equals 5lbs..?
Overall arrow length also has an affect on soine requirement. Every inch over 28" is going to require an additional 5lbs. of draw weight.
Conversely, every inch under 28" will reduce the spine requirement by 5lbs..
For skinny, low stretch strings, add 5lbs. Foe large or multiple string silencers, subtract 5lbs..
Adding weight to the front of the arrow, as in heavier points, adds to the spine requirement. When you get over 145 grs. you will need to bump the spine by 5lbs..
Dynamic spine> Remember we said that dynamic spine is affected by dynamics? Well it is. Here's an exampple.
Carbon arows are the in thing right now. But, a .660 spine carbon should have EXACTLY the same defledtion or static soine as .600 spine arrow of any other material. Now, carbons are measured over a 28" span, sing a 1.94 weight, but even with that, if you spine test a crabon arrow and wood arrow, over the same span. using the same weight, they should "bend" or deflect the same amunt. The difference with carbon is that due to the material, they have the ability to revover, or straighten out faster than wood, or aluminum either one.
Converting the spine of an aluminum or carbon arrow to match the spine of wood arrows (remmeber carbons and aluminums are measured over 28" and wood over 26") simply take the deflection number of the carbon or aluminum and multiply it by .825. For example, a .600 spine carbon conversts to .495 when meausred over a 26" span. That equates to 52.5lbs..
That's the basics of spine and deflection. The individual nuances of the archer really can't be taken into account in a discussion like this.
But typically, people who have a very clean, dynamic release (and that doesn't mean your hand goes flying off your face) require an arrow that is slightly stiffer than a person who has a more passive release. The less disturbance that YOUR relsend release method has on the stirng, the stiffer the dynamic spine requirement. If you shoot with 2 fingers under, you likely require a stiffer spine, You are creating less string oscillation because you you are cutting the interference of the fingers by 30%. If you shoot with a thum ring, you can do the same thing. Release aid..same deal.