EE was standing on face side of LW belt at -1 block directing his EEs while pulling end of belt they'd just installed a splice on. They'd attached a side grip on face side & pulling w/DC winch. EE knelt from a position 8' inby the grip instructing winch Op. to keep pulling when grip slipped off belt striking the bottom of a belt stand then EE's lt forearm & the top of his hard hat.