Employee picked up jackhammer, he was using to punch out sag mill bolts. he hooked up choker to jackhammer so crane could pick it up & take the weight off while he was using it. He picked up jackhammer before the crane got the slack out, and the crank hook pulled & struck his left elbow. He did not report until 2/19/11-his day off, went to Dr.& resulted in a chipped elbow.