User blog comment:IamJakuhoRaikoben/Chapter 396: Air, Review./@comment-41.136.39.177-20140815063524/@comment-4829046-20140815064933

It's like the second container of magic that all mages have in them but are unable to access it due to body limitations, same principle as in IRL really. However Ultear can use her Arc of Time to forcefully release it, giving their bodies the ability to use that untapped energy. Now with Erza, Ultear commented that nothing happened. The reason I believe is cause she understands her body better and has more resilience and maybe she could already access Second Origin without undergoing the drawback to the body.

Long story short, Natsu and Co. had Second Origin forcefully activated and were using it ever since while Erza had more casual access and preferred to use it only when necessary cause I believe she already had an immense amount of magical power from the getgo(S-class mage, duh)

At least that's how I viewed the whole thing. You can look at this page if you want.