Two employees were operating the RRII roof bolter in the #7 entry inby spad #4931 on the 3 left section. They had scaled roof in the area then set the ATRS preparing to install roof bolts. EE was attempting to mount the man-up station; placed his right hand in the buckboard to assist him when a piece of rock approx. 1'x1'x2' fell striking his right hand.