You can spontaneously cast a spell of the element you have mastered.
Prerequisite: Elemental mastery class feature.
Benefit: Choose one spell of the element you have chosen for your elemental mastery class feature. You can now spontaneously cast that spell by sacrificing a prepared spell of equal or higher level. The spell you choose must be in your spellbook. When you gain a level, you can change the spell you can spontaneously cast by picking a new spell of the element you chose for elemental mastery.
Source: Complete Mage, page 42.