DRIVER WAS TIGHTENING LUG NUTS ON TRUCK UNIT #3011 WITH A 3/4 DRIVE WHEEL RATCHET. THE RATCHETS LOCKING MECHANISIM BROKE CAUSING IT TO FREE WHEEL. THE DRIVER WAS APPLYING PRESSURE ON THE RATCH ET WHE THIS OCCURRED. DRIVER STATED THAT HE FELLDOWN CAUSING AN INJURY TO HIS BACK.