An employee was guiding a trash pump that a crane was moving into place for a concrete pour. The employee had to climb onto the trash pump base to release the main block hook from the wire rope rigging. As he grabbed the rigging with his right hand, his footing slipped and he grabbed the top frame of the trash pump. In his efforts to keep from falling, he pulled the rigging (still attached to the main block) toward him, which caused the hook to amputate his left little fingertip.