The commercial pilot reported that he was preparing for a flight instructor practical examination, with a flight instructor onboard acting as a student pilot. After demonstrating several normal and short field takeoff and landings, the commercial pilot began to demonstrate a soft field takeoff. The commercial pilot reported that he "smoothly applied full power" and verified that the engine instruments were operating normally. He further reported that shortly after liftoff, while the airplane was in ground effect, the left wing dipped, and he attempted to correct the lateral bank with right aileron, however the left-wing continued to drop. Subsequently, the flight instructor stated, "we're stalling!" as the airplane touched down on the left side of the runway in a nose high, left wing down attitude. The airplane veered off the runway and the right main landing gear collapsed. During the runway excursion, the right wing sustained substantial damage. Neither pilots reported any preimpact mechanical failures or malfunctions with the airplane.