Set during the Wraith attack.
When Emma touched Regina to get the hat started, Emma's magic touched Regina's magic. Emma turned into a giant fox and both Emma and Regina fell through the portal.
Regina's magic activates Emma's transformation. Regina can cast different spells on Emma to change her form. The fox transformation has levels and is different than the magical forms. For Emma to first progress onto another level, her feelings have to change from tolerating Regina to being Regina's friend to finely loving Regina. When Emma finely realizes she loves Regina, then she can change forms but only if Regina loves her back.
Some sort of connection is made so that Regina and Henry can understand what Emma says when she is a fox.
Wile Emma is a level 2 fox, she finds an abandon fox kit and takes it in. After a fox is born, it can't see, hunt, hear, or defend its self. Now the fox kit thinks Emma is its new mother. According to the little fox kit, that would make Henry its brother and Regina its mother too. Another magical connection happened and Henry and Regina can now understand the fox kit. It is assumed Emma can always understand foxes.
Foxes are a type of dog. Ruby and Emma become good friends. And foxes only mate for life. So for Emma to mate with Regina, it is equivalent to getting married. Although they don't know about that till way after the fact that they made love.
LEVEL 1: Emma turns into a regular small fox.
LEVEL 2: Emma turns into a big red fox. A big enough fox that Regina can ride on her back. Emma's ears are much pointier and longer. In this form Emma can only use healing magic.
LEVEL 3: This is the level that Regina can cast her spells on Emma to change Emma's form. Emma can also use healing magic on this level. This big fox is silver with light blue streaks on her chest, tail, and forehead.
Emma can give Regina a limited ability to see in the dark like a fox can. For a short period of time, Regina can have the eyes of a fox.
FORM 1: Regina launches her fire ball at fox Emma, and Emma turns into a purple flaming fox. Since the flame is Regina's magic, it will not burn her. That way Regina can safely ride on Emma's fox flaming form.
FORM 2: Regina casts a shadow spell on Emma. Emma can now blend into any shadow and can teleport from one shadow to another. Emma fur turns black and has a wispy like quality to it.
FORM 3: Regina conjures a storm and Emma feels Regina's magic from the storm through the air. The magic seeps into Emma. In this form, whenever Emma bites someone, she releases electrical impulses into said person or being. The electricity comes for the lightning or the static electricity in the air. Because Emma can use the static electricity in the air, she can use this form from a natural occurring storm.