At 1733 CDT, a Swearingen SA-227-AC was substantially damaged when it landed gear-up at San Antonio International Airport. The instructor and commercial pilot student were not injured. The student was undergoing first officer training for the Swearingen SA-227-AC. The pilots stated that they were completing the last approach and landing for the flight. The instructor told the student to execute a no flap landing due to a simulated hydraulic pump failure. The student established the airplane on the approach and called for the Emergency Gear Extension Checklist. The instructor stated that the gear could not be extended until landing was assured. The landing gear was not extended. The student continued flying the approach and retarded the power levers to idle to reduce the airspeed, and the gear warning horn sounded. The instructor placed his right hand on the gear extension lever. As the airplane reached the target airspeed, the student moved the power levers forward to increase power, and the gear warning horn ceased. The instructor removed his hand from the gear lever without extending the gear.