User blog comment:PitchBlack696/Monster Hunter Portable 3rd/@comment-3149269-20101208001924/@comment-2040723-20101208011239

It's one of the Jinouga armor's skills, all right; the skill is called Determination. When the arm glows like that, that means you get a temporary boost to your Affinity. It seems to be triggered by something, but I don't know exactly what that might be