Employee was using a 3/4 ton come-a-long to remove a 40' roll of 60"" mine belt from a 975. The belt was lifted high enough for clearance, and then was being lower to the ground when the come-a-long released & free wheeled until the belt hit the ground. EE was holding onto the slack end of the come-a-long and his arm was jerked upward thus staining his left shoulder.