https://www.youtube.com/watch?v=O7zewtuUM_0
You get the point in the first 3 minutes, but its a fun video. We needed some more nerd content here.
Awesome video. Byron's a living legend.
I shoot archery (compound) competitively and I teach classes on the tuning aspects. They left a bunch of things out, if you're interested. The physics behind it is pretty cool.
Measuring the spine is just the first step in what he's gotta do to get consistency down. He already knows what spine he needs based on the setup he's using and hours of testing.
1 - The draw weight and the draw length of the bow both factor into how much power is being used to push that arrow down the range. Higher draw weight is obvious, but a lesser known thing is the draw length. The longer the string is pushing behind the arrow, the more power you're putting behind it. The more power behind the arrow means you need a stiffer arrow (and vice versa).
2 - The weight of the arrow point (tip) goes hand in hand with the spine of the arrow. When you shoot, the energy from the string is being passed through the arrow which is being resisted by the arrow point. Picture what would happen if you put the arrow against the wall and began to push on it - the harder you push, the more it flexes. The heavier that arrow point is, the more it's going to resist, which means the arrow will flex more. It's a time consuming process of finding the right balance.
3 - Each arrow has a stiffer and weaker side. You can index each arrow to predict which way the arrow is going to bend to increase consistency. With a compound, I always index my arrows to flex up. A long bow would be out to the left or right depending on what hand you shoot. This is getting nit picky, but required if you want ultimate consistency.
Also, if you noticed - the compound shooter's arrow flexed vertically where the long bow flex horizontally. That's because the compound shooter is using a mechanical release aid to draw/release the string resulting in a cleaner nock travel. The long bow is a finger release, where the string is essentially rolling off his fingers which creates more side to side nock travel.
