Increase weight in the front - weaken the spine (think of it as more to push against so shaft "bends" more)
Decrease weight in front - stiffen the spine
Not technically correct and nitpickers would tear it up but, to keep things simple, that's the effect.
Decrease weight in the rear - weaken the spine
Increase weight in the rear - stiffen the spine
Actually... I disagree. Weight on the back end of the arrow has no effect on spine. Less weight on the rear end increases FOC and so improves stability and gives the effect of stiffening spine. Naturally, adding weight to the rear has the opposite effect.