The pilot reported that he was practicing the procedures for making approaches to back-country airstrips, and was flying in approach configuration about 40 to 60 feet over a mountain ridge. Although the wind was light and variable, just as the flight was crossing the ridge, the airplane encountered a downdraft which resulted in it suddenly losing altitude. The pilot stated that before he was able to correct the situation, one of the airplane's main gear tires came in contact with the snow-covered terrain, which resulted in the airplane slowing down and flipping over onto its back. The accident sequence resulted in damage to the top of the rudder and to one of the wing lift struts. The pilot reported no mechanical failures or malfunctions with the airplane at the time of the accident.