D&Dtools1

Benefit

If you bestow a negative level upon a spellcasting creature, and that creature loses a prepared spell, you gain the ability to cast that spell once (as if you had prepared it). Treat the spell's effect as if it had been cast by the character who prepared it (including caster level, save DC, and so forth). You need not have the requisite ability score to cast the spell (for instance, you need not have an Intelligence of 13 or higher to cast a fireball drained from the mind of a wizard).

The Spell remains in your mind for up to 1 hour. You can have a maximum number of stolen spells equal to your Charisma bonus (minimum 1); any spell that you would gain above this number are simply lost.

This feat has no effect on spellcasters who don't prepare spells (such as a sorcerer, who simply loses one spell slot for each negative level bestowed as normal) or who have no spells prepared (such as a fighter, or a wizard who has cast all her spells).