A flight instructor was providing flight instruction to a commercial pilot leading to a tailwheel endorsement. Toward the end of the instructional period, the flight instructor demonstrated wheel landings, which the pilot receiving instruction was able to perform satisfactorily. The flight instructor reported that while executing another wheel landing on Runway 17, the commercial pilot started to lose directional control, overcorrected, and used excessive differential braking. The tailwheel-equipped airplane pitched forward and subsequently nosed-over, coming to rest in the inverted position. Runway 17 was reported to be 4,000-foot long by 75-foot wide. The winds were reported as calm and the asphalt runway was dry. The flight instructor added that the directional control could have been corrected with an application of power rather than by brake application. Additionally, the flight instructor reported that there were no anomalies or malfunctions in the airplane's flight controls or tailwheel steering system.