On July 13, 1999, approximately 1845 mountain daylight time, a Cessna 172N, N5396D, was substantially damaged during landing at Centennial Airport, Englewood, Colorado. The commercial flight instructor, his student, and the passenger were not injured. The airplane was owned by a private individual and operated by Wings of Denver under Title 14 CFR Part 91. Visual meteorological conditions prevailed for the local instructional flight which originated approximately 75 minutes before the accident. No flight plan had been filed. The flight instructor pilot said that while turning downwind to base in the traffic pattern for runway 17R, he pulled the throttle out to give the student a practice engine out emergency landing. The student "over shot the centerline, and steepened up his bank to correct back." At the same time, the student pulled the control yoke back, and the airplane's decent rate accelerated. The flight instructor said that he "pushed the throttle to full power and tried to level the wings." The airplane landed hard approximately 10 feet left of the runway, and subsequently veered across the runway and off the right side. The engine firewall was buckled and the cockpit floor boards were diagonally wrinkled; and, the tail bulkhead and the right wing outboard upper skin were wrinkled.