The text on the ability doesn't reflect its actual effect.
For reference, the ability text is: "Reduces elemental resistances of all enemies by 10%."
What it really does is increases the damage of all incoming magic attacks by +15% (THM/BLM/CNJ/WHM/ACN/SMN/SCH damaging spells).
The way testers found this out:
Elemental attacks via PGL/MNK/ARC/BRD/DRG (fists, DoT/circle, dive etc.) do not see increased damage at all. However, it does increase damage dealt by non-elemental "unaspected" attacks from ACN/SMN/SCH. This means the enemy resistances aren't affected at all. The increase in damage was also static on all targets tested on which means it applies a direct magic damage taken debuff.