User blog comment:Yxz2014/Possible big plot hole??/@comment-73.55.144.176-20140930000838

I remember him transforming after Armin started screaming. Maybe a person can only use their powers the first time if someone they care about is in trouble? Kind of like you get that weird adrenaline rush if you're scared. Maybe it works in the same way, but at a greater amount? Other than that, might be a slight oversight.