On June 16, 2003, at 1905 Pacific daylight time, a homebuilt McCarty Avid Magnum tail wheel equipped aircraft, N64KM, registered to and being flown by a commercially rated flight instructor, and accompanied by a student pilot, sustained substantial damage when the aircraft nosed down on landing roll on runway 33 at the Renton Municipal airport, Renton, Washington. Neither occupant was injured. Visual meteorological conditions existed and no flight plan had been filed. The flight, which was personal, was operated under 14CFR91, and had originated at the Renton airport approximately 1830. The pilot reported in a telephone interview that upon landing rollout, approximately one-third to one-half down the runway the tail started coming up and he was unable to stop it. The aircraft slid to a stop in a nose down condition and both occupants exited the aircraft. Winds at Renton at 1912 were reported from 310 degrees magnetic at 9 knots and the pilot reported that he experienced no mechanical malfunction with the aircraft during the event. The pilot subsequently reported (refer to attached NTSB Form 6120.1) that he was occupying the right seat and a student was in the left seat when, at two miles out on final he announced to the student "my airplane." The student responded "your airplane." During the landing roll the student applied brakes resulting in the aircraft nosing down.