On July 16, 2001, at 1620 hours Pacific daylight time, a Burkhart Grob G 103 Twin II, N4805R, was substantially damaged when the glider broke in half during a hard landing at the Minden Tahoe Airport, Minden, Nevada. The glider was operated by Soar Minden, Inc. The flight instructor and his student pilot were not injured. Visual meteorological conditions prevailed during the dual instructional flight conducted under 14 CFR Part 91. The glider departed the Minden Tahoe airport at 1616, and no flight plan was filed. The flight instructor was interviewed by an investigator from the National Transportation Safety Board. He stated that he and the student were practicing landings at the airport when the accident occurred. The instructor said that on their second circuit in the landing pattern, the student was flying the glider within acceptable limits. He was controlling his airspeed and sink rate properly on long final. The glider was approaching runway 30 when they noticed another glider that was much lower and needed priority for the runway. The instructor directed the student to do a sidestep maneuver and land on runway 30 left, a parallel dirt runway. The student began a left sideslip to line up on the parallel runway. The instructor stated that he was controlling the slip properly, and was controlling his sink rate with small applications of the spoilers. During the landing flare, the student applied full spoiler while still 1 to 1 1/2 feet in the air, at approximately 60 knots. The instructor was unable to overcome the rapid nose down pitching moment and the subsequent increase in sink rate. The left wing was slightly lower than the right due to the previous slip maneuver. When the glider landed hard, the left wing made contact with the ground, resulting in a ground loop to the left. The instructor believed that no damage had been done to the glider, but upon exiting the cockpit, he noticed that the tail boom had cracked just ahead of the empennage structure. The instructor stated that in the future, he will place his hand just behind the spoiler control lever and restrict the aft movement to an acceptable position for the flight conditions.