While setting up to cut the straight of #3 entry at 17+40, the miner operator backed the miner out to sight it in when a rock measuring 2'x1'x4"" fell from the roof striking the back of his left hand which was on the remote box for the miner (the rock also broke the box's handle). Initial x-ray indicated bruising only; follow-up determined that a bone in the wrist was fractured.