Sap is an interesting Spell that reduces the target's current MP pool. Their MP is reduced to 1 divided by the number of successes plus 1. In other words, it reduces MP to 1/2 with 1 success, 1/3 with two successes, and so on. The Spell is bugged in that it only acts on the least significant byte of the target's MP, so targets with 256 or more MP are not affected correctly.
Because Monsters who cast Spells tend to have large MP pools, and Spells are relatively cheap to cast, Sap is of limited usefulness. It would be great against the Red Soul in Kashuan Castle, except that boss absorbs all Elements including Mind. If you get it to really high levels, it can be useful against Spellcasters (as long as they have less than 256 MP), but it's not a high-priority Spell.