EE was installing a right-side, A motor V-Belt guard on a HP400 Crusher. EE hung a 44"" X 37"", 20lb piece of guard onto the fastener bolts. As EE was lining up the bottom right-side fastener, the guard came unhooked from top left fastener. This caused the guard to swing down and pinch EE's left hand between the guard and angle support, bruising EE's left hand.