You are so familiar with one spell that you can cast it even when you have other spells prepared.
Benefit: Pick one spell you have taken Spell Mastery with, or one spell from a domain you have access to, to be your signature spell. You can channel stored spell energy into your signature spell even if you haven’t prepared it ahead of time. You can cast your signature spell by “losing” a prepared spell of the same level or higher.
Special: This feat may be taken more than once. Each time it allows you to select a different signature spell.
Adventurer’s Handbook: Genius Guide Volume 1