The employee stated that EE was on the operator side of the right bolter on #2 section. As EE was backing out of 6R, EE spotted a piece of draw rock. The rock looked as if it went back into the rib, so they chose to bolt it to the top rather than try and pry it down with a slate bar. When EE pulled the starter out, the rock fell and broke on the mast, striking EE's left forearm.