THe easiest way to explain it, is that anything currently under the effects of magic, cannot cross parma (without penetrating).
If you are under the efects of a spell (say stat boos) you cannot cross parma, but if your sword is not under the effects of a spell, it can.
Simmilar with rocks, if you use Re/Te to "shoot" a stone, it is being propelled with magic and won't cross parma. If levitate a stone above a target, then cancel the spell to drop it on him, you don't need to penetrate (no longer under the effects of magic), however, you do need a Finnesse roll to aim.