Employees were hanging a horse head on a well. The injured employee was on a pumping unit, waiting for the horse head to be lifted with rig blocks, with a safety retractable line attached. The line got caught in the blocks and pulled the employee off of the pumping unit, causing the employee to fall 10 to 15 feet before the safety line engaged. At the point of engagement, the safety line snapped, and the employee fell to the ground.