According to the flight instructor, the student pilot and flight instructor were repositioning a Robinson R22 Beta to practice "quick stops." The student pilot was air taxiing the helicopter downwind at 30 feet agl, when she began to slow the helicopter. The flight instructor then took over the flight controls and attempted to do "a left pedal turn" so he could point the helicopter into the wind for landing; however the helicopter did not respond and started to descend. The flight instructor then lowered the collective "slightly" and increased engine power "slightly" to stop the descent without result. He again tried to raise the collective "slightly," but the helicopter continued to descend. As the helicopter was "getting pretty low," the flight instructor turned the helicopter to point the skids in the direction of travel. The front of the skids then made contact with the ground, dug in, and the helicopter nosed over. No preimpact mechanical malfunctions or failures of the helicopter were reported by the flight instructor.