Employees were working to unload a power line pole from a pole trailer and place it on the pole rack on the digger derrick truck. The injured employee hooked up pole tongs that were attached to the boom rope of the digger derrick to one of the poles on the trailer. As the digger derrick picked up the pole, it slipped out of the tongs and landed on the leg/ankle of the injured employee resulting in a broken left leg and ankle.