An arrow's actual spine deflection is the arrow's measured resistance to bending.  Basically ... an arrow's actual spine deflection tells you how stiff the arrow is.  For proper safety and best performance, the arrow spine must be matched to the output of the bow.  Arrows which are too stiff or too limber will not fly well and will degrade the accuracy of your bow.  Arrows which are dramatically underspined (way too limber) can even present a failure hazard.  So selecting the proper arrow spine is very important, but it can be confusing.  Manufacturer spine sizing systems may be arbitrary (100, 200, 5575, 45-60) and may or may not reflect the arrows' actual spine deflections.  And some manufacturers list spine sizes in ascending order of stiffness - others in descending order.  So you must often cross-reference the arrow's advertised spine size with the actual spine deflection your bow requires.    

According to the modern standards (ASTM F2031-05) an arrow's official spine deflection is measured by hanging a 1.94 lb. weight in the center of a 28" suspended section of the arrow shaft (not to be confused with the old AMO standard of 2 lb. and 26").  The actual distance the 1.94 lb. weight causes the shaft to sag down is the arrow's actual spine deflection.  For example, if a 1.94 lb. weight causes the center of a 28" arrow to sag down 1/2 inch (.500").  Then the arrow's spine deflection would be .500".  Stiffer arrows will, of course, sag less.  More limber arrows will sag more.  So the stiffer the arrow is, the LOWER its actual spine deflection measurement will be.  The more limber an arrow is, the HIGHER its spine deflection measurement will be.  See actual arrow specification data before selecting an appropriate spine stiffness for your bow.  If you need help, call our pro-shop at 877-410-7811 or email us for assistance.

I'm still confused.  Give me the full lesson from the top!