On a drilling rig, a floor crew rigged a block and tackle assembly to the top of one of the V-door guide posts and used the air hoist to attempt to rotate a 48-inch drive pipe string. The string could not be rotated fully into position and when one of the floor roustabouts (Floorman) went to check the connection, the V-door guide post suddenly broke off from its weld. Propelled by the cable tension, the V-door guide post jumped across the floor fatally striking the Floorman.