How much the arrow bends during the shot is determined by how much force the bow applies to the back end of the arrow, how far off center the arrow shelf is, how stiff the arrow is (spine), and how clean your release is.
Arrow stiffness or spine is measured in pounds and gives you a ballpark guess for matching arrows to your bow. Arrow spine is measured by supporting the arrow in 2 points, 26″ apart and hanging a 2 pound weight in the center and measuring how much it bends.
For example, if the arrow bends .520 inches, that spines for a 50 pound bow at a 28″ draw. Does that mean if you have a 50 pound bow and you buy 50 pound arrows that they are matched and will fly like darts? Not at all! This just gets you to a starting point. Much is left to do and understand before that’s going to happen.
A common misunderstanding is confusing arrow spine with arrow weight. They are not related and are two different things. Don’t fall for the marketing ploy that so and so’s bows are so powerful they require a 10 or 15 pound heavier spine to shoot well. This is not true and in reality has less to do with the power of the bow and more to do with how close to centershot the bow is, as we will see later.