EE was re-attaching the skirt board and liner bolts on the conveyor belt in the #1 Crusher, 6th level down. While pulling on a wrench tightening a bolt on the skirting board, EE lost EE's hold on the bolt and slipped forward, extending EE's left leg, and fell backwards. When EE slipped, EE attempted to catch self, but EE's left leg came out from under EE.