User blog comment:Marco the Marine Admiral/After watching Chapter 316, I bet that.../@comment-5036235-20130126172821/@comment-5668006-20130126223005

It was Kagura's sword that made that huge shockwave throughout the city, NOT Kagura's strength or she could have done it with it sheathed. Also, Erza is obviously stronger but she had never created shockwaves that huge with her swords. Remember when Gajeel ate the grimoire heart dog guys sword? That was the first time he used iron god sword bc of the magic in that guys sword. So Kagura's sword is obviously superior to other swords thus it would juice Gajeel up similar to how the flame of rebuke fired up Natsu.