You are able to quickly modify your mastered spells when you cast them trickily.
Benefit: Whenever you cast a spell that you have mastered with the Spell Mastery feat as a tricky spell, you may spontaneously apply a single metamagic feat (other than Silent Spell or Still Spell) you have to that spell. You may not apply a metamagic feat which would normally raise the level of the spell Greater than the highest level of spell that you can cast. (When applied with the tricky spells ability, Silent Spell and Still Spell do not increase the level of the spell.)
Undefeatable: The Collected Feats Sourcebook, Copyright 2009 – 2010, Louis Porter Jr. Design, Inc. Undefeated, Copyright 2011, Louis Porter Jr. Design, Inc.