3 men were going to move 10 pc of 12"" x 40' urethane pipe with a WA 500 loader. 1 man was giving the loader operator signal to move bucket against 1 piece of pipe that was separate from the others. As the loader hit pipe, other man was standing on top of pipe. He fell off between pipe & pile of pipe pinning his ankle & falling backward breaking his leg below calf & above ankle