The certified flight instructor (CFI) stated that he was preparing the student pilot to solo in a Cessna 140. The airplane touched down uneventfully on runway 36, a 4,700-foot-long, 75-foot-wide, concrete runway. Toward the end of the landing roll, the CFI instructed the student pilot to make a 180-degree turn, and back-taxi on the runway. As the student pilot began a right turn, the airplane traveled over a hump in the runway, which startled the student pilot. The student pilot applied right rudder in an attempt to move away from the hump, and the CFI perceived him as over-controlling the airplane. The CFI inputted left rudder pedal; however, the student pilot had also depressed the right brake. The airplane continued to turn right, and the left main landing gear traveled off the runway, into a berm. The left wing subsequently struck the berm, and sustained damage to the wing spar. Examination of the wreckage by a Federal Aviation Administration inspector did not reveal any preimpact mechanical malfunctions, nor did the pilots report any.