I' ll tackle this one for yeah. Spine is in relation to a arrows strength, and flexibility. The spine has an effect on accuracy based on your arrows size to bow poundage. Spine of an arrow comes from its wall thickness, length, diameter, and grains. Arrows bend when released from the string. Too much bend results in underspined arrow. Not enough bend & forgiveness results in an over spined arrow. For more information go to www.eastonarchery.com Click on tech room at the top of page. Download Easton tuning guide.