"Conjuring magic is not an intellectual endeavor it's emotion. You must ask yourself why am I doing this? Who am I protecting?" Rumple spoke as if he were remembering things of the past. With such a softness and emotion to his words.
Emma stood there listening to his words, concentrating oh her emotions. Her need to protect those that she loves. But within this moment, she sees the unexpected. Her mind flashes back to moment where she stood behind Regina as she tried to make the hat work. Seeing the wraith in the distance, hoping it didn't get passed her parents. Hoping that Regina could make the impossible possible. So she could live another day to see Regina.
Emma thinks back to her urge to give comfort to Regina in that life threatening situation. The moment she truly accepted that she cared for the brunette and not just because Henry wanted Emma to protect Regina. But because Emma held strong emotions for the brunette.
She thinks of her gut instinct to touch the brunette's arm. Making magic surge between them both. And the look in Regina's eyes as the brunette turned to the blonde. As if Regina felt the emotions too. Emma's emotions.
Emma can feel the magic coursing through her veins as she tapped into her need to protect. She wanted to protect Regina from killing someone, to keep her from going back on everything she worked for.
What does the final trick is that Emma thinks of when the wraith got passed Charming and lunged at Regina. When she wanted to protect Regina with her life, no matter if she died.
With that the protection spell was cast, causing the white magic to forge a barrier around Gold's shop.