You have the ability to leave a portion of your arcane potential untapped, allowing you to adapt to unforseen challenges. When you prepare spells, you may choose to leave an amount of your additional spell Power unselected. This amount cannot be higher than your Level.
At any point in the day, you may take a quick action to prepare a spell. The Power of the spell cannot be higher than the spell Power that you have reserved, and the prepared spell's Power counts against this reserve of spell Power.
Thern is a Level 2 Mage who can prepare five spell Power per day, beyond his two Power 1 and two Power 2 spells. He prepares a Power 1 and Power 2 spell from his additional pool, and places two of his spell Power in reserve.
Thern's party encounters a locked door, and the party's Rogue completely fails to unlock it. Thern uses one of his reserved spell Power to prepare the spell, Open The Way. This is a Power 1 spell, so he has one Power of spell reserve remaining.