While greasing a deadline sheave, a Roustabout (who had never been to the crown before) placed his arm through the fast line sheave for stability. At the same time the rig mechanic on the rig floor asked the Driller to move the blocks to enable him to grease them. The Driller forgot the Roustabout was working in the derrick until after he stopped the drum clutch immediately after starting it. The grease gun was caught in the sheave, wrenching it from the hand of the Roustabout. The Roustabout suffered slight bruising to the forearm and the grease gun fell to the ground just missing a Company Representative. THIS INCIDENT HAD HIGH POTENTIAL AND COULD HAVE RESULTED IN AN AMPUTATION OR A SEVERE BLOW TO ANYONE WORKING ON THE FLOOR.