ORIGINAL: Roskoe
Bob H - it is my understanding that adding weight at either end of an arrow will cause it to flex a little more in the middle. The FOC, however, will increase by adding weight to the front or removing it from the rear.
the way I was taught is opposite ends have opposite effects. So think of it this way, put a weight up front and the arrow will take more of a "push" to make it move, hence will flex a bit more then with a lighter front end. This is weaker spine.
Put a heavy weight in the back and again takes more of a push, but the resistance is on the back end hence it flexes less.
It works in my head [8D]and matches what I've read. I have also plugged it into software to pick shafts and it shows the same. though the weight difference in what you can do on the back end is smaller than the front end.