The pilot reported that he was turning the glider from base to final at about 400 feet with full flaps and spoilers extended. On final about 100 feet above the ground, the pilot decided to change his flap setting from 20 degrees to 10 degrees as there was a cross wind. When he went to move the flap handle, the handle slipped from his hand and instantly changed the flaps to 10 degrees, which caused the glider to lose lift and nose down. The pilot then pulled back on the controls which caused the glider to stall and collide with the ground. The glider was substantially damaged and the pilot was seriously injured during the accident. The pilot reported no pre-impact mechanical malfunctions of failures with the airframe or the engine that would have precluded normal operation.