Spine and Archer's Paradox
Every arrow shaft has a degree of stiffness called spine, which is its resistance to bending. Bending, known as "archer's paradox," occurs when an arrow is released from the bow. The forward thrust of the string causes the shaft to bend in one direction and then react in the opposite direction as it speeds downrange.
Spine strength must be matched to bow draw weight. If your arrows are too lightly or heavily spined for your bow, the "archer's paradox" movements will be extreme, resulting in poor arrow flight and loss of accuracy. (It's better to err on the stiff—too heavily spined—side.) Arrow manufacturers publish selection charts that match bow weights to proper arrow spine. Your local archery shop will help you match your gear.