Employee stated: He had been cutting bottoms for a new belt drive. He was beside the miner when a piece of coal rib sloughed off. He had cut 2' of bottoms but had not undercut the coal. The piece was 4""-6"" thick X 2' tall X 5' long. The toe of his boot was under the apron when he jumped and twisted to avoid the falling coal. He broke the navicular bone in his right foot.