Sifen stabbed through his arm with a needle and began bleeding into his arm and making it numb (My first of many times giving blood this happened to me, shit sucked and my arm was brown and purple for a week because my vein was bleeding INSIDE of my arm) and he passed out from blood loss since he neglected to take the needle out. While passed out and floating a large piece of boat hit his arm and crushed the veins and the nerves between the piece he was on and the one that hit him, ceasing any blood flow and therefor oxygen to his arm. That and then they water around the limb for so long kinda sped up the whole being dead process and the chocobos clawing around in the water tore it up and filled it with salt water and such. And now it's dead woohoo!