As you prepare your spells, you wisely save a spell slot which you are able to quickly fill later with a spell you need.
Benefit: When you prepare your spells for the day, you may leave a spell slot that you would normally use to prepare a spell empty. Later that day, as a full-round action you can fill the slot with any spell that you would normally have been able to prepare in it. You may then cast that spell as if it had been prepared all along.
Normal: Normally filling your unused spell slots takes a long period of rest rather than a full-round action.
Undefeatable: The Collected Feats Sourcebook, Copyright 2009 – 2010, Louis Porter Jr. Design, Inc. Undefeated, Copyright 2011, Louis Porter Jr. Design, Inc.