I often get asked or looked at strange when first mentioning finding the first dynamic spine in and shaft. So here is something I have learned to pass along.. Your shaft has a natural bending point that during the launch the shaft will want to flex towards the weak or easier found side of the bending point., but what many don't realize there often more than 1 bend of the shaft, often there are 2.. 1 of the bending points that feels more like going over a pot hole which you dont want and the other like a gradual dip in the road, that is the one you want mark as the dominate bend, why, you want more predictability in the launch which translates to better consistency as finding the dominant bend on all your shafts can help reduce and or eliminate culling of the arrows. With vertical bow the spine location is more ideal placed up or down since the energy of the bow is generated vertically. It does not matter the direction up or down just that it is and the arrow leaves on the same bend. For cross bows this becomes more critical since a rail is now involved. The spine location mark on weaker side always faces downward with cock vane in rail. What this does is during launch cycle the bolt's natural bend can cause the bolt to flex up ward off the rail or minimize pressure as apposed to if the bend was unkown and happened to be reverse the flex would apply pressure downward on the rail causing more pressures, friction adding more resonance into the shaft causing over all performance loss.. For better performance spine location can be a huge deal and it can aide to better gains complimenting the null point of the shaft whether in vertical or cross bow types.. The dynamic bend is important and spine stiffness can be critical as we want the shaft to react ( bend ) during launch of the bow.