Spine refers to the stiffness of the shaft. It affects the way the shaft flexes in its flight. If you make the arrow front heavy the arrow will flex more and keep flexing or oscillate too much during its flight. A underspined arrow (too flexible for the application) will cause the arrow to impact the target with its tail to the left. So the anwer about foc and spine are affectively the same. A crazy foc like the having the balance point towards the rear will cause erratic maybe dangerous flight but a small change in foc should cause a change in impact.
Try one or a couple of arrows of a different spine or a different weighted point to see what happens. You can go to a 160 grain tip which will make the arrow flex more to see if that goes left or right of the point of impact. This will affectively change your spine as would the length of the arrow.
I hope this isn't too confusing.
Easton's tuning guide will bring some enlightenment to the subject.