Good explanations.

First Focal Plane - Crosshairs grow and shrink with magnification, but relative to the target are always the same. So if you have matching turrets/reticle, the adjustments work as you expect.

Second Focal Plane - Crosshairs never change size with magnification. Meaning relative to the target, they change. So the adjustments will correspondingly not adjust correctly, except at one magnification.

Simply shooting and adjusting power, will give a impact shift on a POS scope. But that is regardless of focal plane.