The 598-hour flight instructor was giving the143-hour private pilot a currency ride in a single-engine airplane, when the engine lost power. The flight instructor reported that while on the downwind leg of the traffic pattern, the private pilot "consciously and vocally" announced that he was performing a "GUMPS" check, which was an abbreviated pre-landing checklist for Gas-Undercarriage-Mixture-Propeller and Switches. The private pilot checked the fuel selector (Gas), and noted that it was set to the full forward position (right tank). He then "twisted" the fuel selector handle to the "full back" position and inadvertently turned it to the OFF position. Shortly thereafter, the engine lost power. The private pilot turned the airplane onto final approach, and told the flight instructor that the engine "was not producing power." The flight instructor took control of the airplane, and it subsequently impacted trees and terrain short of the runway. Examination of the fuel selector revealed that it could be turned to the OFF, LEFT and RIGHT positions. When the selector handle was pointed full forward, the RIGHT tank was selected; when the selector handle was pointed straight up, the LEFT tank was selected, and when the selector handle was pointed toward the rear of the airplane (full back) the OFF position was selected. Incorporated on the fuel selector cover (placard) was a safety button that was designed to keep a pilot from unintentionally turning the fuel selector to the OFF position. The pilot would have to physically depress the button so the fuel selector handle could bypass it and be moved into the OFF position. The private pilot stated that he was not aware of this safety feature and was adamant that he did not depress the safety button when he moved the fuel selector during the accident flight. According to an FAA inspector, who performed an examination of the airplane, the fuel selector handle was found in the OFF position; however, when he moved the selector handle through its full range of motion, the handle moved freely to the OFF position without having to depress the safety button.