Damage is dealt to MP rather than HP.
Mana Shield (ＭＰすり替え? or ＭＰすりかえ, MP Surikae?, lit. MP Switch), also known as MP Switch and Damage > MP, is a recurring ability in the series, primarily found in the Ivalice Alliance games. It transfers a portion of the damage the user takes into MP damage.
Mana Shield is a reaction ability learned by the Time Mage class for 400 JP. Upon taking damage from any source, the damage is taken out of the user's MP instead of HP unless the user has 0 MP before damage is taken. If damage taken is greater than the amount of MP the user has, the user's MP is reduced to 0.
Converts HP damage to MP. (Doesn't work if MP is 0).
MP Shield returns as a reaction ability learned by the Blue Mage, Tinker, and Arcanist classes for 450 AP from the Mirage Vest. If damage dealt to the user is greater than the amount of MP they currently have, any overflow is taken from the user's HP.