Spine Defined: Spine is the measured flexibility/deflection of an arrow. This allows you to match specific combinations of draw weight, arrow length and bow/cam design to the most forgiving arrow spine. An arrow must flex to be accurate & forgiving, not too much, not too little. Having the correct spine is EVERYTHING to superior arrow accuracy.
Stiff rear portion allows for the nock end to remain straight as it exits the bow, eliminating nock travel.
The middle 1/3 of the shaft harnesses and controls the flex of the arrow, both as it exits the bow and as it flies down range.
Stiff front portion seemingly eliminates broadhead oscillation.