With FFP, the reticle doesn't appear to change size (in relation to the target image) when we change the power setting. This is a benefit when we use the reticle to hold over, hold off for wind, or call shots, or attempt to range or calculate size with the reticle. All these things can still be done with the scope on any power with a FFP scope.
2nd FP scopes need to be set on a specific power for the reticle to subtend the given amount. If we change powers, the reticle size (again in relation to the target image) changes too. Not necessarily a bad thing in some cases, because if we've got a 1 moa scope reticle and it feels too cluttered, we can dial down the power and make the spacing increase to 2 moa (just an example). Another benefit of 2nd FP, if we use hold over technique; we can adjust the power to where certain marks on the reticle match our drops at a certain distance. This is something that can't be done with FFP.
FFP = When target appears larger (as we increase the power), reticle appears larger too. Visa Versa if we decrease power.
2nd FP = as target appears larger, reticle stays the same size, thereby appearing to decrease in size relative to target.
Hope that helps.
